I can't fathom playing that many different games at the same time - I'm lucky if I have time to play 2 or 3 concurrently, and even then it'll be months before I get through them.
Well in my case it's not a question of playing them at the same time, nor is it a question of having the time for them all. It's about the convenience of having them installed and not having to constantly uninstall and re-install games due to lack of storage space. I have 131 games in my Steam library and if I was to install
everything I would need
at least a 4TB drive, which I don't have (currently only have 2TB main drive and a 1TB backup drive, that's it). Heck, Steam in and of itself is convenience, you don't have to go to the store to buy retail / physical copies of the games you want; just download them. The principle of having most of my preferred games installed and not having to constantly put them aside (uninstalling) is the point here. When I come home and I want to play Witcher 3 I don't have to tell myself "Well damn, I should uninstall Fallout 4 first, and then reinstall Witcher 3 to play it for just a two hours session; and then do exactly the same again for Fallout 4 tomorrow or next week end just for a 3 hours session". That, to me, is the part that would drive me crazy.
That's not even including my GoG's and Origin's libraries (at 28 games each), along with my retail games (around 40). There's also some MMORPGs that aren't part of digital platforms like Guild Wars 2, Blade & Soul or TERA (well TERA has been on Steam for a few months now but it hadn't been the case since its launch and for a period of about 2 years and a half, possibly more) that I'd like to keep installed but I just can't. And there's also Blizzard's own "platform" with their own launcher (I own D3, have Heroes of the Storm and both Wings of Liberty and Heart of the Swarm).
In other words, I want to keep my "favorites" installed at all times. Still, I don't play them all at the same time, obviously. But I just can't fathom having a meager 512GB SSD, or even just a 1TB one (but that's way too expensive anyway) and then dealing with installing just a dozen games or so. And I sure won't just go out and buy like six SSDs just to have 3TB worth of "faster" drives at the price of both arms and legs. I have absolutely nothing "against" SSDs as a new technology. It's faster? Ok, that's cool. Just wake me up when I can actually buy one (that's affordable) that has at least 2TB on it (heck at this point I'd ask for 4TB, even on HDDs I need that and actually I'll have to buy one in a month or two).