I seem to remember reading that both walking upright and the widening of the birth canal to accommodate our larger crania may have shifted the locations of the human females’ reproductive parts somewhat. It could be that on a quadruped the clitoris is positioned so it gets enough stimulation without any special attention being paid to it.
Humans are also somewhat unique in reproduction - their fertile time is hidden (there’s whole books on the whyfor of that), so nature has motivated both males and females to engage in reproductive activity all the time, not just when the time in ripe, so to speak. Pleasure is the major incentive for this.
I guess we don’t know what it “feels like” for other animals, but obviously for many reproductive activity only happens in season, in response to the correct cues. I presume this means it’s more like an itch to be scratched than an intensely pleasurable experience.
OTOH, apparently male monkeys will masturbate in the right circumstances; although I have never run across reports of females doing likewise.
Somewhere in the world there is, or once was, a guy who would sidle up to female porpoises and, er, stimulate them. No I don’t recall where I heard this.
John Lilly wrote of a female researcher in his dolphin communication project who did this regularly with a male dolphin. IIRC it was in his book Communication Between Man and Dolphin.
(Lilly also later became a proponent of mind-altering experiences, experimenting with isolation tanks, LSD, ketamine, and other drugs. In one of his later books, he acknowledges that he gave LSD to his research dolphins to see what would happen. But he didn’t seem to have much to say about what actually happened – apparently the dolphins’ overtly visible behavior wasn’t affected too much, as best I could tell.)