Well, setting aside the fact we don't KNOW if they haven't yet (Yakuza PS4 is probably exclusive, but I doubt they'd have had to pay for that one given that Nagoshi is pretty confident where his target audience is/is going it seems), why the fuck would anyone want third party releases to be exclusive exactly? That shit needs to stop surely?
My guess is that they considered first party investment to be a more worthwhile way to spend their cash, which is absolutely the right call. Third party exclusives will likely end up on other platforms eventually, but first party output will always only be where you want it to be... so which would you rather spend money on? Obviously Microsoft needs third party exclusives to offset their barren first party library at this point in time, but Sony obviously do not.
Oh... Plus, not buying timed exclusives has the added bonus of taking away Microsoft's favourite PR play from this gen: Betrayaltons! MS tried to get one of these early with the Insomniac thing... then no one cared. If Sony had signed that FFXV exclusive deal they supposedly had in front of them, they'd have had a re-run of FFXIII 360 all over again at some point.
Staying away from full game exclusivity and instead getting extra content to make your version of the game look a bit more appealling is the better move for them right now... and may end up proving unnecessary down the line if and when word starts getting around about PS4 versions of games being the best you can get on consoles anyways.