Higher audio rack Quality makes sense for turntables, and other things with moving parts. Amplifiers which seem to be a bit more sturdily built and if they had casters I don't think that would impair the sound of a 150 pound amplifier.
Although if it does, it would sure be interesting to know why that's the case.
I use an Anchor sound rack with casters for Mono blocks and it's solid as a rock.