Limits on how fast you can stream and/or decompress data from the disc. Better devs (e.g. Naughty Dogs) do spend the time to properly plan ahead and make sure the game can run off the disc entirely. Others don't, or bite more than they can chew and couldn't do it even if they wanted to.What happened to. Putting in a game. Turning the console on. And playing the fucking game. Why do we need to install shit that is on the disc/game card/whatever.
That's why I kinda laugh at the next gen consoles tauting SSDs. Not only is it not new, but you can bet your ass that'll just push devs even more to do required installs. Why bother with that slow as mollasses BD drive when you got an SSD? Nevermind how big those will actually be, with games requiring upwards of 100gb now. Some fanboys going on about how they'll stick in NVMes, even though when I opened up my PS4, what I found was a white label drive. So much for "cutting-edge next-gen", they aren't going to stick a 2TB drive in them and still be affordable to you, kiddo.
I 'member the old Neogaf days with Sony drones praising the PS4 and how the sky's the limit with its specs. Heck, remember those demos? You could take those, replace the "Not in-engine footage" fineprint with "Rendered with real-time raytracing", slap a PS5/Series X logo at the end of them, and people would still eat it up as the hot next-gen crap even if the rest hasn't changed.
The fact that they even focused on named specs is silly and sticks out from the last generation. "The PS5/Series X got a Navi GPU in it!11!1". Neat, Big Navi is around the corner, your console is still at least 9 months away from release, and it's already outdated.