There may be several reasons why sperm didn't evolve to be temperature tolerant. The first is likely that they would have suffered reduced motility just when it

There may be several reasons why sperm didn't evolve to be temperature tolerant. The first is likely that they would have suffered reduced motility just when it's needed, reducing fertility rates. Another reason might be that maintaining the testicles at body temperature increases the risk of infection. There are very few purely sexually transmitted diseases, likely due to this temperature difference. Those same sexually transmitted diseases rarely cause harm beyond the genitals, and in fact these diseases are far more harmful to the female reproductive system than the male (a woman with undiagnosed gonorrhoea or chlamydia is essentially guaranteed to become sterile, however in men it either takes longer, or they may never become sterile - note that a male can lose a testicle to injury or disease without becoming sterile or losing fertility). It's likely that there were more advantages to any male with dangling gonads over retracted. The reason why our testicles require constant temperature control (and independent control) is probably a more intriguing question, which IMO likely has a lot to do with our method of motility. A dog's testicles are unlikely to be affronted by the sun, however any naked male human has a serious problem when walking toward the sun. This likely wasn't a problem when we were semi-erect (in the evolutionary sense, not the other sense) and had hairy gonads, however why did modern humans develop wholly unprotected gonads? Think about it, our testicles don't even have protection against the sun! Dense hair provides protection against heat stroke, so did humans already have protection for our genitals before this became a problem? IE did our invention of clothing happen early enough that it actually changed the development of our sexual organs?