Because the company believes he's worth it. People have mocked Vince for years for paying his over inflated salary for the handful of dates each year for awhile. Because the WWE itself makes a shit ton of money essentially with Brock having no effect on it. I mean the Smackdown deal itself is $1 billion dollars. As Cornette joked , Vince can change NOTHING now and still never worry as his company is in no danger. So he can keep shoving Lesnar out there. Keep having him bury guys and do long title reigns with no one caring
AEW hopefully has enough common sense to not bury the guys they are building for the future to Brock Lesnar in this act if they did sign him. Or hand him a title and rarely get title defenses from it.
Its 2019 , Brock Lesnar's act is stale as I posted above. You have seen it for 5 years. At this point I'm sure Vince will do it for another decade.