Asking this question in this way (demanding an explanation) frames it so that people will instinctively feel defensive. There's no shame in being a virgin. You aren't entitled to an explanation from anyone about why.

Because I never had sex.

Because some people apparently prefer to spend their time reading stupid questions online.

It's a pretty good movie, might watch it again.

There could be lots of reason for not having sex:

Asexual

Low sex drive

Never found a person they could connect with like that

Being an awful person / falling into the internet incel category

Different motivations in life that they didn’t feel the need for sex

Medical problems - this could cover a lot from psychological issues to physical issues

Fear of pregnancy / failure of birth control

Strict religion

Nervous/anxious/shy/awkward personality (like in the film)

If you're that old, you probably want to be, and that's totally okay.

Too busy watching the 40 Year Old Virgin