How does a guy who hasn't started a game since high school make it to the NFL?

I'm just wondering here and maybe it's a simple answer but I just can't seem to figure it out. How does Matt Cell not start a game ince H.S and still make it to the NFL? He sat behind Leinhart and Palmer at USC which means he never saw the field and somehow he made it to the NFL. Now a guy who hasn't started a meaningful game since H.S. is going to be starting QB for the New England Patriots. How does this happen?