Most "asexual" people have sex and date. A small percentage have medical issues or trauma; most are just lying for attention.

They just portray themselves as special because they perceive their libido as being lower than others, or they split hairs trying to separate sex from attraction from romantic love, etc. insisting they only feel a portion like they're sexually colorblind or something.

(Which begs the question - if you don't feel it, how do you know what you're not feeling?)

In reality it's probably just that a lot of people identifying as this are young, and relatively inexperienced, and are basing their identity off of a single feeling or interaction, possibly not even IRL. They got a crush on an anime character and now think they're "demiromantic", or they watched too much porn and now they think they're ace because they think normies all fuck within minutes of meeting each other.

Almost every "ace" person says they'll have sex and even feel love under the right conditions, so it follows that they're just still figuring out who and what they're attracted to.