Why aren't multiplayer console games cross-platform-playable?

Hi,

I used to play FFXI on my PS2/3 with people playing on PCs and Xboxes, so there doesn't seem to be any technical reason for it, and I would imagine that what the games are sending out over the network would be pretty much the same data for all platforms concerned.
Does MS refuse to let PS owners "interact" with their players, to encourage people to buy their games "in bulk"? Does Sony shut out Xbox users for the same reason? Do they both prohibit it? Are devs just too lazy to make it work? Is there some other reason?

Please help.

I didn't find the right solution from the Internet.

References:-
https://arstechnica.com/civis/viewtopic.php?f=20&t=131125
3D Product animation studio

Thanks!