Vast number of reasons there, really, it could be down to anything.
As previously mentioned, if they weren't as good looking as they are now when they were in school, which is a vicious place, then it's quite possible they've still got their own demons from there and they've not entirely overcome them. Or, they've felt repelled by seeing things from both ends of the spectrum, from being overlooked because they weren't beautiful enough when they wanted to be seen past that as a person not an item, only to jump across the spectrum to the point where they're wanted only for their bodies, and still being seen as an item and not a person.
If the above is true, it could be that they still see themselves as that undesireable, the geek who looks in the mirror and dreams of having those killer pecs, blind to the person that they've become. Could well be that they still feel ugly inside for it, that opening up to somebody would only make them feel worse, or make the reciprocant feel terrible and thus make them feel worse for it.
It could be down to a sex drive; if they've always been like they are, or thereabouts, it could just be an interest thing as not all guys are as bothered by sex as we're portrayed, just like the varience in women's sex drives. Or it could be that they're simply waiting for the 'right' person and they're not too interested in doing it outside of love, that the passion just isn't there and it's meaningless without it. Everybody gets different things out of sex, sometimes you need more than just the orgasm to enjoy it. Of course, a virgin perhaps wouldn't know that entirely until they've tried it, but they can still desire it only with that love, passion, drive that you can only get sometimes in a relationship.
Relationships could be an underlying issue, that they're just genuinely unlucky and that everybody they meet who would be good for them is already taken, or they've just not bumped into them - the world is both a terribly small place and a lonely place at the same time. They could still be haunted from previous relationships and closed that part of them up, concentrating on other things before confronting those demons, or rather, waiting for that right person to confront those demons with.
Their confidence with the opposite sex, past a platonic stage, could be next to zero through either inexperience, past cases, both, or just genuinely having absolutely no 'game' nor drive to earn such a confidence. Or they could simply overthink things, question motives and drives as opposed to simply losing themselves in the moment, something I'm guilty of myself. Or that they're in love with somebody, or have a crush on somebody, and reject everybody else because it wouldn't be the same, rinse and repeat through their years with different girls?
It could well be the above, that they have the looks to "invite them in", but don't possess the personality or the drive to seal the deal, as it's not always a pure physical thing with both genders; a lot of the time for guys it is, same for girls, but if they're not open for a ONS then they're not going to act on the signals and instead look for something else, which leads to them perhaps not having the strong personality needed, or that they lose them into the friend zone too quickly, or get assumed to be gay if they're good looking but have little interest in sex.
It could well be the above actually, that stigma of being 'too' good looking only to be seen as being gay for not being too flirty, overly interested in girls, might have given them a stigma that they can't seem to shrug off. I've been told many a time myself that people think I'm gay, or bi after I'd dated my ex, because I've never been that guy who chased girls, rather be the one who steps back and sees the guys fall over each other at her feet, but it gets mistranslated as uninterested (understandably so); that and I do admittingly act a bit camp at times with my interests and the likes (since when has astronomy been seen as gay?!).
At the end of the day it could be anything, you just don't really know. The important thing is that everybody gets different things out of sex and have different desires out of it. Some of us want sex, doesn't matter who with as long as they're attractive, some of us want sex to feel desired, some of us want sex for the rush of passion, some of us just aren't that interested in it for whatever reason, some of us don't see the appeal in ONS.. so on and so on.
tldr; We all get different things out of it, but it's not the be-all and end-all; your sex life shouldn't be the defining point of who you are. That gets lost on a lot of people unfortunately, but like I say, everybody has a different sexual drive and a different want out of it.