Why in human males is the prostate such a troublesome thing? Because by the time the prostate becomes a problem, males have generally done any breeding they're going to do, so there is no advantage to natural selection to improving it further. Is it optimal? Definitely not.
Presumably it is (taking the wide view) probably a good thing that evolution doesn't find global optima or there would be far less ecological diversity.
It's not really that it doesn't matter, just that there are several different options to allow good enough fertility.
If sperm has to be stored/generated at a temperature lower than 36°C, then external testes are a solution to that, but a lower body temperature works as well. Developing enzymes that work good enough at a higher temperature also works (apparently what birds have done). And maybe just accepting a lower fitness of sperm cells works if the animal produces more of them.
Hippopotamuses have a low body temperature of about 35°C, so internal testes work for them.
A hippo doesn't care much about looks ;)