They have the room to accept the rush of vehicles. The issue is Disney doesn't want to pay to properly staff for that scenario.
If people want to stand around for hours by Boardwalk that's their business. Disney just doesn't want to deal with people peeing in cups because they don't want to leave a line and other idiotic things people do in those situations. I get it. But to say they have no options is wrong. They just would rather take the course of least resistance which also happens to conveniently be the course of greatest profits.