Its mostly longer development times with some project cancellations. Most of it is weird qualifiers that other PlayStation consoles didn't have to go through. Perhaps a side effect of Microsoft purchasing a few publishers?
Ratchet and Clank or Sly Cooper count as first party for the PS2 but Helldivers 2 isn't for PS5. People counted MediEvil Remake, Wipeout Omega, and Everybody's Golf as first party last generation. Many aren't even counting Everybody's Golf: Hot Shots as a first party game this year. Sony owns these IP's so its not a case like Silent Hill 2 Remake that can go multiplatform after a specific amount of time.
And from what I understand, Nintendo has a few of these arrangements as well, but people don't really nitpick what counts as first party for them.
