Lots of reasons.
There isn't always an unlimited supply of experienced people applying for your job. A person can only have 1 job at a time. Granted the sex/job analogy sort of breaks down here because a man CAN actually be in sexual relationships with multiple women at a time unlike a job. But then it just comes down to a matter of, would you really rather be side girl #14 for an experienced guy than have a virgin to yourself (who stops being a virgin after 1 time anyways). Besides that, there's also
Everyone starts with 0 experience. Every experienced guy only got to become experienced because at some point, a woman said "Sure I'll have sex with you even though you've never had sex before". If every woman had the same paradoxical "I won't have sex with any man unless he's had sex before" then humanity would go extinct.
Human beings are more than just the functions they perform. If a guy had lots of positive qualities is it worth it to turn him down solely because he'll be a virgin for 1 time having sex and then never again?
Having "experience" doesn't automatically mean a guy will be any good at sex. A guy might have had lots of sex before just by being hot or a smooth talker but that doesn't mean he's learned anything.
Having no experience also doesn't automatically mean a guy will be bad as sex either.
The guy with no experience might have more potential to be good at sex than the guy who's had it before, you'll never know if he never gets a chance.
What's so awful about a round of subpar sex anyways? It has no permanent consequences. I can't imagine how you would see this as a major problem unless you're someone who has constant 1 night stands with different guys and then never sees them again and you just don't want your 1 night with this particular guy to be boring.