Do you really find it that difficult to find these?
Steam has a tag devoted to Local Co-Op. There are
2,983 matches:
Steam Store Hub
store.steampowered.com
Alternatively, you can go to the SteamDB.
Steam tags are also here, but there are more ways to find games here. Under the "Number of Players" you can choose "Co-Op" or "Online Co-Op" or "Shared/Split Screen" or probably what you want, "Shared/Split Screen Co-Op". The last shows
10,004 titles on the SteamDB. You can then refine the results a million other ways. For example, you can set it to only games that have an 80% positive rating overall with at least 1K votes. Then you can checkbox the release year. Doing this, and picking only games released from 2020 or later, corresponding to the 9th generation of consoles, there are now 148 results. If you go back to 2013, to include the 8th generation of consoles, it's now 300 results.
In the Xbox app, you can go the "Store" tab on the left, then at the top of the front page for the Store pick the section "All PC Games". Once there, under the "Number of Players" drop-down filter menu, you can filter for either "Online Co-Op" or "Local Co-Op". For the latter, there is
484 games. You can then further refine that search with the drop-down filter on the right, "Capabilities", to only show those that offer "Cross-Platform Co-Op" if you like. Basically all of these are available on Game Pass. And many that are available on Game Pass that aren't brand new will be available with PS Plus.