The beam steering takes about 10 milliseconds. That means effectively the beam can't be steered at all during use (imagine you steer the beam to one user, send a few packets to them, then steer the beam to another user, send a few packets to them, and repeat - thats a 20 millisecond jitter the users incur - completely unacceptable).

That forces the beams to remained trained on a single user or area (covering a group of users).