All guys want to have sex, even the relationship material guys.
However they notice the nonrelationship material guys (fuckboys) get most of the sex.
A lot of girls say that when they meet a guy who is hot but not relationship material, they will be more likely to have sex with him right away but when they meet a guy they see as being more stable and reliable they will take things slow and have him wait for sex. Girls often end up developing feeling and falling for the fuckboys even though the girls never really saw them as relationship material.
So the fuckboys get easy sex with a wide variety of women while the non-fuckboys get ocassonally sex and when they do get it they have to take the girl on dates for like 1-2 months even though she put out on the first date for the fuckboy.
As a result a lot of the non-fuckboy guys want to become fuckboys because they get better results.
Because nice guys realized the increase in attention they get and made a sub conscience decision to change. If more nice honest average looking boys got attention I promise fuck boys will disappear. Another reason is good looking guys realized they could have any girl they wanted and they got them so average guys started having that mentality and it worked in their favor.
- Gender segregation, and as a result lack of empathy between the two sexes - Women wanting guys to be "masculine" even though it has a bunch of negative traits as well - Women being socialized to believe that abuse is affection (look at the sales rate of 50 Shades of Grey)
Basically, it's because that's what works with the promiscuous women.