Because it is owned by Lawrence Stroll who has a lot of money. I don't think the budget caps cover wages.
He doesn't like working too far from home which will detract from the lure of Ferrari, as Aston Martin are about ten minutes further from where he lives than Red Bull are. Then again, he is 65, so this could be his last hurrah.
I think it is more about the politics of the team to be honest. He doesn't like fuss as it makes his own job harder. Have you read his book? It is a free audio book on Spotify if you haven't.