QuentinGeorge
Legend
That is just not true at all. There are plenty of evil gods of myth/religion
Not in the way the worship or nature of "evil" deities is in D&D. I mean, Ares was an unpleasant individual, but not a satanic sort of force, more a personification of mindless violence. The titans represented a sort of primordial evil (although not completely), but they were explicitly outside the pantheon of gods. None of them had priests or clerics cackling in temples dedicated to evil. The most destructive group of worshippers in the Hellenic world were in fact the Bacchites, and no one would consider him "evil".
Set was originally one of the patron gods of egypt, and only ended up demonised, so to speak, thanks to his adoption by the Hyksos invaders. After that he was certainly an adversary, but also active worship of him effectively ended. There were no "priests of Set" wandering around causing havoc. The malign figures of a religion might be given an offering, hoping to draw away their attention, but no one would worship them in any organised way except for the crazed lunatics which most rulers would be driving away as a matter of urgency.
Kali wasn't evil, and Loki only seems as such in our minds because many of the interpretations of Norse mythology were coloured by early Christianity filtering into Scandinavia which changed the significance of him and Ragnarok. We are viewing a lot of these classic religions in a Christian-inspired lens, making us search for the "good" and "Evil" members of the pantheons.
A lot of D&D's problems with evil gods stem from its refusal to have much in the way of post-Antiquity religions (monotheisms and the like) but at the same time wanting to have organised religions complete with priestly hierarchy and big buildings in the middle of cities. D&D settings present polytheisms but then have them act like monotheism, complete with priests of Bane operating unhindered in civilised centres. No one would tolerate religions of open evil - the priests would be pelted with rotten fruit and driven from every town and city.
Clerics are based on Templar Knights more than anything else, so it seems strange that we are so resistant to actually having them in religions where they might make sense.