I've been playing video games for 30+ years, and for 27, 28 of them, I was almost exclusively a solo player. Between console games where I didn't have "gamer" friends except my neighbor, who really only played old school Smackdown and GTA3, and PC games where I never really competed online, games for me were strictly a solo escape.
Over the past couple of years, though, as my life has changed and my understanding of myself has too, I've finally started playing more with friends in co-op situations. ARPGs (Lost Ark, Last Epoch, DIV), PlateUp, and my latest addiction, multiworlds.
Maybe it's me finding out something everyone else has known for ages, but it turns out that video games are a lot of fun as a communal experience. It also turns out, though, that video games aren't as much of a solo experience for me as they were. I find it hard to just boot up a game and play for me, because a lot of the joy I get out of the games now, comes from the sharing of time with friends.
So, it's not that games have become less fun. It's that I'm finally finding out how much more fun it can be to share a good time with people you care about.