That all resets about once a year when a new expansion gets out.

You invest into fortnight the same as you invest in anything, you specialize your skill for this one game, your social circle revolves around it etc. You don't need gear to invest in a game

Gear does, true. But there is a lot more personal attachment to an account than gear. There are achievements, mounts, pets, guilds, social connections/friends, etc... Also during it's peak the reset was much less of a thing (Wrath was the first expansion that started the gear reset per tier trend and it was also the peak of the games player count).

WoW expansions don't come every year.

These days there's a soft reset every 5-6 months when a new raid tier comes out rather than only once per expansion. For the last few years they've tried hard to make it so that when someone who played years ago gets the urge to give it another try they can actually make it into the endgame content before that urge passes.