There are two reasons for a PSU being placed at the bottom; 1) better cooling and 2) better balance for a case as it puts a bulky item low down and hence lowers the overall CoG.
A bottom-mounted PSU can draw cool air directly from outside the case, over the internals and then back out. A top-mounted case can only draw warm air from inside the case - this will increase the ambient temp of the PSU and lead to a shorter lifespan.
A modern, good-quality PSU won't radiate much heat. They typically have efficiency ratings well north of 80-85% now, which means a minimal amount of the power drawn is wasted as heat. So that's not an issue. In terms of impact on other components, there are plenty of other heat sources inside a PC well in excess of the level of heat a PSU will radiate - primarily GPUs. Yes, mobo components are sensitive, but they're designed to work in a PC environment and the heat level associated with this. A good PSU won't appreciably contribute to this.
Now, cheaper PSUs may be different - they typically won't be as efficient, won't have the features of a good-quality PSU and may very well radiate more heat. But again, not very much. If a PSU is radiating significant heat, radiate that PSU right out of your system before it goes horribly wrong for you.
The vast majority of case manufacturers put PSUs on the bottom as it is seen as the best design option. In a competitive market, with an enthusiast audience who value low temps and high framerates, they will look for every advantage to make their product the one to buy. If placing a PSU elsewhere was a better option, we would see it coming through in case design.