So, there's a few ways to come at the title question - "Why are old PCs considered lame and boring?"
Obviously, this in the context of computing collectors, not normal users that just want something to get the job done.
The first thing is simply that old PCs are arguably the same thing as modern ones, so why collect an old one, when a modern one's the same platform? I'd argue that that's not exactly true, but it's very hard to point to something and say that that is when the PC fundamentally changed (partially because it took years before a change trickled down and got used widely):
- PCjr? OK, that had a decent run in the form of the Tandy 1000, but VGA and Sound Blaster utterly displaced that, it didn't really contribute to the modern day PC platform I'd argue (although for retrogaming, it's actually a big deal, because you really can argue that this is different from the modern PC... except compared to the competition, it was rather lame)
- AT? Most of the AT's actual innovations for end users were in the form of speed, and the peripherals that mattered existed for the 8-bit channel too. And, even the 286's greater addressable memory, while it got used by OS/2, was largely unused until Windows 3.0, and that really wanted a 386.
- DeskPro 386? It was just used as an AT with go-faster stripes for, what, the entirety of its production run? (That is, as that model - there were other 386-powered DeskPros later on, of course, that were used for at least some of their 386 functionality (386 Enhanced mode, if nothing else).)
- IBM PS/2? In the grand scheme of things, as far as what actually became part of today's PC... it added HD floppies (a logical extension), a mouse port (not important), VGA (very important), and later, accessibly priced accelerated high resolution graphics (8514) to the architecture... and IBM themselves backported the first three to the XT architecture, either at or soon after launch. (Model 25/30 for the HD floppies and mouse port, and they sold an 8-bit VGA card because of the 25/30 shipping with MCGA. Everyone else cloned the 8514/XGA's general concepts as part of their SVGA extensions of VGA.)
- Sound Blaster 1.0? As a gaming and multimedia platform, that could be argued to be a truly major change for the PC platform... except it was evolutionary on the AdLib.
- PCI? Faster, better configuration... but it still had to coexist with ISA, and the ordinary 32-bit 33 MHz variant was already becoming obsolete (with AGP being needed for high-speed graphics, and PCI-X being too expensive for consumers) before ISA was gone.
- USB? This was a huge innovation in retrospect... except nobody really cared until Apple pushed it.
Everything else wasn't really a sea change in the platform, I'd argue. But, those presumed sea changes either ended up being dead ends (PCjr), or being gradually merged into the platform (literally everything else on the list).
Basically, the PC platform isn't demarcated by major changes in the platform like the Amiga is (1000/500, then 2000/3000, then 1200/4000), or for an even stronger example, like the Mac is (68k, PPC (and there's a cutoff somewhere in the G3 era for where OS X doesn't run well at all, and you should really just run OS 9 - OS X being a sea change for the platform), x86, and then within x86, one hard cutoff for the EFI32 to EFI64 changeover (that's abandoned hardware that, on the PC side, is still usable for Windows 7 or 10 today)). So, that muddies the waters as far as what an old PC is, and contributes to a perception that they're just lamer versions of what you could buy in any store today. (Now, on the software side of things, there's some firmer demarcations, largely tied to the mass adoption of Windows in the early 90s, then Win9x, then every major consumer NT release. But, even then, the continuum gets blurry once you get into the Win9x era, as the platform - even today - shares a lot with Windows 95 both in terms of user interface and the software that runs on it, even if under the hood is completely different.)
Another factor that's been brought up is the state of the PC platform in the 1980s. A PC in 1992, or 1995, was a very different beast from a PC in 1987, after all, and the Amiga curbstomped a 1987 PC at gaming and multimedia applications. (Productivity, well, the PC was already entrenched as a standard by then.) Some of it may well be bitterness at the crappy XT clone with CGA or Hercules and PC Speaker audio beating the Amiga in the market place... although that's not the PC that beat their Amiga. The PC that beat their Amiga was probably a cheap MPC1 rig (or at least the same sort of idea - the optical drive might have been left out) with a 386SX, running Windows, that was compatible with everything that the new owner was doing at work, without BridgeBoards or software emulators. Let's face it, an MPC1 rig (especially if it had SVGA) could compete with an Amiga 1200, no problem. Or, it might've been an MPC2 rig in 1994-1995, right as the nails were going into the Amiga's coffin - a 486DX2/66 with a VLB SVGA card, a SB16 or so, and a CD could beat up on an Amiga 4000.
Finally, there's another direction I've seen an anti-PC mentality from - not the consumer direction, but the professional direction. There's people who see the PC as architecturally uninteresting, cheap, underpowered junk, that's clearly inferior to Real Workstations™. (And, nowadays, a workstation is just a PC with ECC and an ISV-certified version of the same graphics silicon that gamers use.) Some of them also will play with home computers, and typically AFAIK those interests will tend towards things like Macs or Amigas - they (especially Amigas) might be cheap junk, they might be underpowered in the 1990s, but at least they're architecturally interesting (read: different from PCs, and not x86). There's a lot of x86 hatred coming from that corner - either more ideologically pure CISC architectures like VAX or 68k, or various RISCs, are touted as being superior. (And, well, they may have been in some ways, but just because the CPU is ideologically inferior doesn't mean that it didn't work well... and a lot of the RISCs had higher memory performance demands due to poor code density, too.) Basically, this actually starts to loop back to Amiga types being bitter that their "superior" platform lost, just coming from a different direction in some cases.
That said, the x86 machines I play with tend to either be unique in some way, or that I have nostalgia for (whether I owned them back in the day or not).