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.

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)

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

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

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

Too busy watching the 40 Year Old Virgin