Well at one point he wanted to drop out to work but Fiona didn't want any of it and said he had to leave if he left school so then Lip left and blah blah blah.
But much of the point of Lip's character is that he is self destructive and has a brilliant mind but isn't maximizing his potential for various reasons.
He also can't just pull money out of his ass just because he is a "super genius boy", he's conjured up plans in the past but in the end you generally need money to make money. He helps with selling drugs out of the ice cream van. His most recent scheme was when he had those kids doing charity work instead working for money that he was receiving, can't remember exactly how it played out but was something along those lines.
Im not concerned about Lip's age or his emotionless sex with Karen. I still can't stop laughing about Carl putting rat poison in Patricks sandwich, the kid tried to murder him! This show does a great job showing how kids learn from the people around them.
They can make a really good spinoff one day where the kids are all grown up.