Yeah, but the problem is that you use Steam to install and play the games. If anybody could play the games that were installed on your computer, then that would require Steam to recognize your specific computer, and only work on your computer...because otherwise you could log into Steam on a different computer, and then other people could play on that PC, and another PC, etc.
With games that just install with a CD or DVD, it's tied to your computer and your computer alone, so anybody who uses your computer can play that game basically.
With Steam, you can install a game on any PC with internet, on multiple PCs, so you can't just let anyone play your installed games, because they're allowed to be on multiple PCs.
I understand why it's annoying to you, but if Valve let users share their games with other accounts, the whole system wouldn't work.