Because it’s awesome!
In all seriousness, I love building retro rockets. I would have killed for 100x better performance in nearly all respects at a reasonable price back in 1998-1999. I get to relive those days with way over the top hardware we only dreamed would ever get manufactured back then.
I’ll readily admit that I never used Win98 (either FE or SE) back then because I migrated to NT 4 for personal use just before Microsoft released Win98. I have a bit of a new-found appreciation for Win98 since then even though I still prefer the NT-based OS’s of the era.
As for modern hardware, I’ll echo some of the sentiments above. I enjoy pushing the boundries of what you can get old software to run on. My most modern Win98 experience was with a Devil’s Canyon platform (Core i7-4790k, H97 motherboard, 16GB RAM [1.7 GB usable], Blu-Ray and DVD Burners, 128GB SSD, and a Radeon x800 XL). It was a fun project and worked exceptionally well for my game and application tastes, but not really all that much faster than my usual Win98 machine that uses an Ivy-Bridge Xeon at 3.6GHz with similar peripherals.
I’d like to try Win98 on something newer than the i7-4790k, but I’m not really interested in buying newer hardware for such a build at the moment (I’m running out of space for cases). Maybe in a couple of years.
P.S. Build whatever you like. It's all part of the fun.