Actually there are some that will... but this is purly academic, I wouldn't recommend them in the 99% of aquariums.
Fish from the Genus Scatophagus (meaning "Dung Eaters") do exactly that... eat poop. The downsided here is they get BIG, start in FW, need to be moved to Brackish, and then need a HUGE marine aquarium as adults (partly because they are a schooling species, get almost a foot long and are ACTIVE). In nature they can sometimes be found around sewage outlets getting a free meal. That said, it isn't their whole diet and they do need other things as well.
There are other detritivores and such out there, but when you get right down to it... they poop too. So no matter how you cut it the only way to get that poop out is with Gravel vacuuming (while it's solid) or water changes (for when it's been broken down). Remember, anything we put IN our tanks needs to be taken out by US in one form or another
. Plus those fish need a more varied diet anyway lol.
Chris