From what I have seen about the entire "Nice Guy" Topic, is much more complicated then a lot of people seem to think it is.
Many of the men or teenagers that act like "Nice Guys" Haven't been in a relationship before, coupled with experiences where they see people who have treated them like crap, has lead to a unfortunate misunderstanding where they think that many women like jerks, as opposed to women liking confidence, strength, success and so on.
This further coupled with more likely than not, being called nice, or being praised for say opening doors, holding doors open, carrying things for other people, has lead to them being nice as a noteworthy trait, something that is uncommon rather than being expected.
It goes a lot deeper, and is a serious problem with my generation, or the new generation, I am not even sure what generation I am part of anymore, (I was born in the 90s), we are treated as children as special, unique, everything we do good and praiseworthy rather than actual accomplishment, it leads over to adulthood, where the only thing they were praised for was being nice, and now they think that is enough to get into a RELATIONSHIP, now many people see relationships as a way to get sex, and it's true many young relationships don't have to be marriage material, it might even be healthy to have someone to help you relax, for both sides.
Further, I am not sure there is any actual proof to show that the "Nice Guys" only want sex for being nice, it is just as likely, they like many people just want companionship, or an actual relationship, (Though as I said earlier, it wouldn't be farfetched to say some, not all want a purely sexual relationship).
TLDR - Not all of the Nice Guys might be after sex and sex alone, and it might be their upbringing that has caused their attitude and mindset.