Currently Online 
A few corrections:
Sony also launched Horizon Call of the Mountain (albeit a VR game).
TLOU Online is still somewhat in development.
Concord is a first party game that is confirmed for next year.
I think there are a lot of issues that are contributing.
1.) Sony is leaning more towards shorter marketing cycles. There are more games that are being announced with months of notice. There are still some with a couple years like Wolverine, but those have been the exception, instead of the norm. I would absolutely expect more announcements for the second half of the year.
2.) Game development is taking longer. Putting out a game every couple years is becoming difficult, whereas it used to be the norm. A single team normally takes like 4-8 years to put out a game now. You can kind of get around that by having multiple projects in different stages of development, but some of SIE's studios don't seem to be doing that currently such as SIE Bend, Sucker Punch.
3.) To a great extent, Sony would rely more on smaller games (they've since closed the studios that made a lot of smaller titles) and also they would publish a ton of third party titles. I know this thread is specifically about first party titles, but if you look at 2006 or 2013 or any year before late PS4 era, you would find tons of published games that are actually made by third party studios. And this weaker first party year isn't actually pretty normal. Like 2016 was pretty much just Uncharted 4.
