It really depends on the girl. Some girls use it to keep guys who like her wrapped around her finger and string him along, making them do their every bidding while waving the possibility of sex in front of them.
Other, non-manipulative-***** girls genuinely want to be friends with the guy because they are uncomfortable with getting into a relationship for whatever reason (for example, just out of a break-up/don't know the guy well enough to make a decision/not sexually attracted to them/interested in another person).
So before you guys try to come up with yet another Overarching Universal Theory Of All Girls (or all people, since this issue can go either way and apply to any gender/sexual orientation).. which I know a lot of guys in teenage years wrack their brains trying to formulate one.. every single girl is different, her reasons will be different from the next girl's, and you should be careful not to treat every case exactly the same.
In summary, if she is a manipulative ***** in general, though, be wary and walk away. If she is the nicest girl ever, don't be hasty to give up. From personal experience (NB: I'm a shy girl who was pretty much asexual and not interested in anyone during highschool), I would have friendzoned my current boyfriend if he tried to push the relationship thing with me too soon. Sometimes it's just a bit scary and interest comes as a shock if you never really considered being in a full-blown relationship with that person.. and sometimes that first reaction is to run away or protect yourself with saying "Eheheh.. ^_^;;.. BUDDY! >_>;;"