The inside of the vagina doesn't have that many nerve endings, if it was a nerve rich pleasure zone, having a baby would destroy a woman 10 times more than it already does.

No amount of sexual skill can make up for lack of nerve endings if all you do is PiV. You need to stimulate other portions of the body.

The penis and the Clitoris are equivalent organs. They both form from the same structures in the womb, and they have similar amounts of nerve endings - the clitoris is more condensed.

As a society, we consider penis-in-vagina sex to be "real sex" and everything else is an optional side thing. There is an incredibly long history of this. This is even enshrined in law in many instances.

if you ever watch porn, it almost exclusively focuses on the use of a penis and ends with the penis ejaculating. From a biological perspective, men have an orgasm and then enter a refractory period and are done with sex

These factors together form a CULTURAL idea that P-i-V sex is the only real sex, and many women just lay there because it doesn't occur to them that it could be better.