The point of first party exclusives is to make money from your store long term. If they make their first party titles available on other platforms, fewer people would buy a PlayStation, which means less long term royalties from store sales.
So you limit the customer base for your first party titles, but ideally you make a ton more on your store fees. That’s the same reason Valve makes first party titles, to get people on Steam, not to make money from game sales.
What they should do is make a handheld that can play PS4 titles. That attracts a different demographic and keeps control of the store royalties. But they really need to make sure it works well, since it’ll be competing with the Switch and Steam Deck (and similar handheld PCs).