This is what is happening:

The venue only holds around 7k seats. The WCC divides those tickets 10 ways and gives each conference member I'm guessing about 600 or so seats at most. They probably keep a few for themselves, or maybe the league office gets some, I"m sure. These have to be tickets to the entire event — all rounds, because they don't know who will advance.

Most WCC schools try to protect these seats by making them available only to season ticket holders. None of them have that many of those however as they are small fan bases.

What Gonzaga fans figured out to do years before BYU was in the WCC was to form groups to buy season tickets at the other WCC schools. That gives them the right to buy tournament tickets, and they buy the max allowed. The venue sells out as soon as they are available. Empty seats happen in the early rounds because those ticket holders show up to watch when their team is playing.