Yeah, there is nothing to gain for them at all. They don't need "affiliations", they get chances to land those companies big stars all the time, and do. So from a networking aspect, not needed.
From a brand standpoint, that only shows their loyal WWE fans that giving these other companies a shot to watch is acceptable. If they lose even small viewership to say, AEW, that's a loss. Everyone knows and watches WWE the most, so you are only giving exposure when working with smaller companies, and not gaining any at all.