Tryna solve a game design riddle.

I play games to finish them. Be that the story or 100% completion. If I'm not making progress, I'm not having fun. This is a known quantity. I've completed more games than I can count or remember. And I've always gotten tired of a game if there's no end goal. I dont play endless multiplayer, I always get bored of it.

Except Team Fortress 2. I've played the shit out of TF2. I have almost 1200 hours in TF2. That's more than my time in Skyrim, Metal Gear Solid V, Borderlands 2, Crash Trilogy, New Vegas, Persona 5, Mario Odyssey, all of which I completed.

Why is TF2 the one endless game I found the motivation for to invest long-term time in?