....Why doesn't Metamask just have it where I can go on a site and enter the login credentials that were created when I created the mobile app?
This happens when you install Metamask on a new device. Do not treat it the same as a centralized platform where you just need to input your password since it's a decentralized wallet.
Needless to say, I am terrified of doing this because I keep hearing NEVER INPUT YOUR SEED PHRASE, and right after following these same steps before (setting up the Chrome extension) I got hacked. And the site I was on before and am on now is metamask.io.
You are on the correct one so no need to be worried there. The "never input your seed phrase" applies only when you're not on the official site.