It’s not. It’s money. Real, Barca, Atletico, Juve, City and Chelsea qualify almost every single year anyway. I genuinely think that if UEFA offered an oil branch of sorts financially whilst keeping the competitive integrity of the competition by ensuring that clubs would have to qualify on merit these clubs would agree.
The closed shop aspect will have no doubt been pushed by the American owners of United, Arsenal and to a lesser extent Liverpool, as the competition for CL football is too fierce in the PL. 4 spots for 6+ teams so there’s always a couple of “big” teams who will miss out.
Naturally it also suits Milan, Inter and Spurs but it will have been driven by the Americans.