Because you can't?
"The "equipment" may be "built right in", but its purpose is not to be used lightly and indiscriminately."
This is just your opinion. You offer nothing to back it up.
"Look around at the sorts of people who have a lot of casual sex. Do you seriously want them to be highly represented in humanity's genetic future"
Yes? Of course? Why ever not? They are humans, after all?
"You're conflating "inexperienced" with "unskilled". I don't think there's much of a correlation."
The two words are practically synonyms. One gains skill through experience. Again, go try and get a job and use that line.
"If anything, people who are overexperienced tend to be shitty lovers, because they've never need to develop any skills."
Is it opposites day? Do you need a dictionary? They have developed skills through experience, which is the only way skills can be developed. You are utterly fooling yourself if you think otherwise.
"If someone's that good, why would s/he have slept with 20 other people and still not found someone who came back and wanted to make it a relationship?"
Because they like sex and feel no need to make it a relationship. You are confusing your own priorities with those of others. They value their freedom and do not want a relationship.