1. Naturally low sex drive 2. They're busy doing other things 3. Sexual repression from parents or religion or trauma 4. Low self-esteem 5. They choose that kind of sex life 6. Boring sexual partners And probably lots more. I'd go so far to say the majority of people no matter thier looks or personality have a boring sex life and pretty much everyone has a boring sex life compared to someone else. Your exciting can easily be considered someone else's mundane.
Well fist a lot of people don't go out and just have sex.
So. e are super shy, or very socially unsuccessful. Like me, I've never dated, and part of that is that my social confidence is very low and it takes me a long time to get used to somebody. And I have never had any girl ever really get into me. And lastly I don't have much to offer, I don't have a good job, I don't know anything about fun things to do, or what to talk about.