PlayStation has the biggest userbase. So on most multiplatform games, like Fortnite, the majority of the userbase play on PlayStation.
If there is no crossplay, if you have to chooe one of the versions of a multiplayer game, most people choose the platforms where all the people (including your friends) are playing. So PlayStation gets benefit from it.
If crossplay gets enabled, PlayStation loses a portion of its sales because some of these players won't be forced to play in the PS version.
In terms of business, crossplay hurts PlayStation and benefits Microsoft and Nintendo.
In addition to this, on the techy side to 'translate' from a network to another (let's say from PSN to XBL or from PSN to Steam) creates an overhead that increases input lag, so for the player iself the gameplay quality becomes a little worse even if may not be something most players would notice.
In terms of business for games like Fortnite, crossplay hurts PS and benefits MS and Nintendo. This is why MS and Nintendo accepted faster and Sony only accepted if the related loss of revenue got compensated.
This is a mail fom 2018, that as mentions there multiple times it talks about UE4 and not UE5. It isn't related to UE5 at all.