The real answer is, computers suck. Seriously, computers are effing awful machines, all developers are hackers, and the only reason things haven't changed is because we constantly lie to ourselves that we're okay with the world.
Passwords suck, no one can remember them, we have way too many of them to remember, and every damn site needs its own special password requirements. And on top of that every site needs its own username requirements too, so there's two things to forget. And email is the universal password, so you want to make sure you use a good password there, but you never really log into your email, do you? The password is saved. So when you get logged out somehow, good luck remembering the password.
And don't get me started on keyboard shortcuts. At least Windows is consistent, quick question: what's the keyboard shortcut on a Mac to go to the beginning of your current line? Ah trick question, it's different for every god damned application! And some applications don't even allow for that without highlighting the entire line!
There's a reason "digital natives" are great at using phones and choose to use them more. iOS and Android were built from scratch and could learn from all of the mistakes of the old systems. In the process, they came up with a ton of their own mistakes (3D touch and long press each do different things? Really? Also TouchWiz is just a bad idea), but consistency and focus are major wins here.
Computers suck and people get scared. Only having one app on the screen at once, only having one way to do things... that's what people like. It's not perfect, but man computers suck. Anything that makes them suck less is a huge win.