Because usually he's been playing most of the game and has been worked into it at that point. A player being able to heat up in crunch time and being asked to manufacture points in the middle of quarters in limited minutes is a completely different thing. He's a player that seems to rely on that rhythmic aspect of the game. If the whole point revolves around him getting free reign offensively off the bench that would assume most likely without other shot munchers there to get in his way. When would he do that exactly and for how long? The 5-10 minutes that Reke/Cousins aren't on the floor? Yeah, that's for Bobby Jackson, not Marcus Thornton if history tells us anything.
Maybe he could develop into that type of player but I think the one he is now is just fine for any team looking for a TRUE shooting guard.
Jamal Crawford averaged 30 min his career playing that scorer role off the bench. Same with Jason Terry the past few years. You speak as if coming off the bench means getting 10 minutes a game or something. Ginobili comes off the bench. So does Harden. So Thornton comes in off the bench in the late 3rd, plays till around the 7 min mark of the 4th, takes a breather and then comes back to close out the game together with Tyreke and DMC, or Brooks/IT and Tyreke/DMC if we choose to go small for playmaking's sake.