Because it is hard to find quality QBs. He has natural skills that a lot of other QBs don't have. If you can untap his potential, then you've hit the jackpot. Most likely, though, that's not going to happen because he needs a lot of playing time just to get down some of the rudimentary stuff a QB has to recognize. But teams waste time on a player like him because there are just not that many great QBs in the league and when you see someone with the talent to become one, you want to explore the possibility as much as you can.
He was a first-round pick because of his running ability and his arm talent, but I agree with you that his experience was worthy of a later pick. Would have had no problem with Dallas spending a fourth-round pick on the player, though, when he was drafted. You rarely find QBs in the fourth round who are ready to step in and start like Dak did. In fact, most of them never become starters. For that matter, a lot of other first-round picks at the position also flame out like Lance did, but most of those at least enter the league with a lot more experience that he has.