Digital foundry had an interesting take on this. Cod makes more than 1 billion a year, and cost probably more each year than any other franchise to develop and maintain. If Microsoft made it an Xbox exclusive, they might cut that 1 billion dollar figure in half, and the franchise might bleed more money than MS would make selling more consoles. In fact, the franchise might go negative.
Basically, they can’t afford to lose the ps5 playerbase.