^^^ A lot of the empty seats you see on MF and TTD are from people being ejected from the coaster. If they sit down, try to buckle the belt and can't pull the slack, they get tossed off the ride. The ride ops aren't going to then call out for single riders. The train was already delayed enough waiting for the person to get buckled in. There's no sense delaying it more just trying to fill a couple seats.
In general, though, I totally agree with you. I've never been to another park (besides Disney) that asks for single riders. So, it's annoying waiting in the queue watching almost full rows of the trains go empty.