I think most of us who build these systems are those who were there actively involved (or at least, admired) in and during the pinnacle of DOS machines and DOS gaming. Most of us were probably students back then, depending on the miracle that our parents might buy us the brand new shiny 286 back then. Some of the components were simply beyond our reach for some.
Now that we have aged, and started working, these components are easier to get (though some are still expensive) and it gets more and more interesting to experiment with the different hardware for use in DOS especially.
For example, the MT-32. It was the "dream" sound add-on for a PC back then. (And still is.) But it was beyond my farthest reach those days. Games nowadays have the same music and sounds, regardless of the sound cards. All windows games sound the same. But not in those days. Games were programmed to take advantage of individual sound components. The Secret of Monkey Island, for example, sounds differently in FM mode and MT-32 mode. It makes me wonder - "wow this game has these sounds and music too!
Even though a real MT-32 can be successfully used in the latest PC with the USB-MIDI cable and Dosbox, some of us tend to still build an actual 486 or Pentium system, based on our nostalgic feelings.
Sometimes, the process of pushing all possible TSRs to upper memory and trying to squeeze out the maximum conventional memory possible from a particular setup, in itself, is like playing half of the game.
Seeing an AWE32 or SB16/Pro/C/MS card and a Roland MPU-401 (or 100% compatible) sitting side by side, pumping out sheer, raw, classic audio goodness into a powered "sub-woofered" speaker setup is something that is unmatched.
Some tend to get everything period specific. For me, since I prefer slimmer and smaller deskspace for a monitor, I prefer to use a LCD screen. Each builder will have his or her own characteristics and preferences.
And finally, if there are means to relive our hobbies of building them, why not. 😀