I did want to mention that, but left it out to keep my comment short. Yes, game development is very difficult and complex. Getting anything working out there is a huge accomplishment for everyone involved.
I have a feeling many companies found that the ratio of work (and thus investment) involved compared to the potential profit generated, especially with predatory MTX added to everything nowadays, means it's pretty much a no-brainer to them to create PvP games rather than co-op ones.
Creating interesting gameplay systems and keeping things fresh for players is (I'd say) undoubtedly more difficult than just plotting players against one another. On top of that, netcode and balancing aren't non-existent in co-op games.
Just take a look at the cancelled Blizzard MMO project "Titan", which was partially repurposed to become Overwatch.