They did release a lot of games in 2018, 2019 and 2020. I think they released 11+ exclusives in that period. Then they also ported many of those to PS5 - either by patches or by full remasters/remakes.
It makes sense that they will release their next batch of games at the tail-end of the generation, considering that they all do big AAA games, which now take 5-6 years.
Similar things happened in the last generation as well: Sucker Punch launched Infamous SS at PS4 launch and then Ghost of Tsushima ~5 months before PS5 launched. This is not a new phenomenon. It is more prominent now because games now take even longer to make.
That is why Sony relies on external studios to make PlayStation Studios games, which, in my and most others' opinion, are still first-party PlayStation Studios games.
You should view them as 1P PlayStation Studios titles as xDev is also involved, which is one of the biggest first-party PlayStation studios. Returnal was a first-party game, even before HouseMarque's acquisition. Just exchanging some money and completing some paperwork to acquire a studio has no tangible effect on the status of the game and does not make it 1P from 2P / 3P -- especially for us gamers.