This exact thread was done about a year ago, but here are some reasons:
Gear between your speakers can screw with your soundstage. Instead of having my gear on the floor I have it all to one side for this reason, but putting it down low gets it out of the way as well.
Many people like to run longer cables from preamps to amps, and shorter speakers cables. If the preamp is on the side (it often is) that leaves the amp in the middle. So why is the amp on the floor....?....well....
...many of these amps are 100+lbs. The Levinson 432 that I had was over 125lbs. It was too big to fit in a rack and I didn't feel like lifting it onto my console-rack anway. The only place it would go was on my short amp stand. And it broke the giant spike off of my amp stand it was so heavy. So it sat close to the floor. Sometimes it was just on a board on the floor, as the board would slide. It was so big and annoying to move that I sold it, and replaced it with a better-sounding amp that weighs less than half of what it weighed. Sometimes this amp is on a low stand, sometimes not. Depends on what speaker cables I'm using, some are short and since my rack is off to the side, it gets put in the middle on a low stand with long RCAs running to it.
Another thing with that amp, it needed a LOT of cooling (as does my smaller Class-A Pass Labs amp). Racks crowded with gear do not offer adequate ventilation for some of these types of amps. The Levinson's convection heat sinks needs basically nothing about it, and little if anything around it, to work adequately. It was a 400wpc amp and it didn't run cool.
People that run mono-blocks especially (and a lot of high dollar stuff involves an amp for each channel) seem to like putting the amps close to the speakers and running as short of speaker-wire as possible.
Also, some people (including myself) just like the look of amps on amps stands. It generally presents less clutter in a room than a big ugly rack. But my room doesn't really allow it so usually everything is in my old mono console that's been re-purposed for stereo gear.
Hope this explains some of it. BTW, my avatar is proof that height does not equal safety from cats.
The final reason is: some people just like it that way. Everybody has different taste. I know I have to remind myself of that when I see a mountain of speakers shoved right up against a wall, speaker that couldn't possibly image or present a soundstage like that, or a preamp sitting right on top of a power amp that needs cooling, leading to the power amp overheating and possibly leading to serious damage, with a turntable on top of both of them for good measure.
I've gotten more use out of the pair of these that I bought than almost any accessory...