Two times WWE Hall of Famer Booker T who is also a former five times WCW World Heavyweight champion and a former WWE World Heavyweight Champion recently revealed why WWE did not work with any other wrestling promotions.
WWE is the only major wrestling promotion in the world of professional wrestling which does not work with any other professional wrestling promotion and does not allow their wrestlers to work in for any other wrestling promotions either. But all other wrestling promotions work with each other.

Booker T On Why WWE Does Not Work With Wrestling Promotions
But this has not been the case in WWE forever. We have seen WWE working with other wrestling promotions before the Attitude Era started. We have seen WWE working with Lucha Libre AAA World Wide in Royal Rumble 1997. A total of four wrestlers from the promotion appeared in the Royal Rumble match of 1997, one of them being the legendary Mil Mascaras.
But other wrestling promotions keep on working with each other. For a recent example we have seen Kenny Omega holding both AEW World Championship and the impact World Heavyweight Championship at the same time. All other professional wrestling promotion work with each other except of WWE.

Booker T who is also a former five times WCW World Heavyweight champion recently spoke on a recent episode of his Hall of Fame podcast where he revealed why WWE did not work with any other wrestling promotions. He said;
“I think contractually, it would be hard to do it over a long period of time with your roster. One or two guys, three or four guys that you want to use in matches, I don’t know. That might work out but having the whole roster work with somebody, I can’t see that happening. I can’t see WWE doing that.

“Even Reality of Wrestling, I can’t see Reality of Wrestling working with another group long term because I’m trying to create my own thing.
“These are my wrestlers. We’re trying to create a storyline and go on this path and make sure our guys stay in this position. It’s hard to do that when you’re working with so many groups. Who’s going to win? Who’s going to beat who?
“It could be a can of worms trying to do something like that. I could be wrong. That’s just my opinion. AEW is doing it pretty well working with Impact, but if you notice, just like I said, they’re only working with Impact with about 3 or 4 guys.”
H/T to WrestlingNews.co for the transcription