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?
As far as I can understand, you generated your Metamask wallet on your phone, and you're wondering why you need to import your seed phrases into your desktop/browser extension. The reason for this is that the metamask secret recovery phrase (seed phrase) is only stored locally on your device, which means the encrypted format of passwords, priv key and seed phrases are only stored to your phone. Thus you'll need to enter your seed phrase again to your desktop in order restore access to your account.
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.
More significantly, if the account you're importing to metamask has previously been compromised, I highly suggest that you should create a new metamask account and make sure you've figured out why your wallet was compromised to avoid making the same mistake.
Metamask link:
https://metamask.io/How to Secure Our Wallet to Prevent Being Hacked!