Why can’t I just use my fingerprint as my password to access everything?

Read this part of the article which explains where you can use it Use Touch ID on iPhone and iPad - Apple Support

and this article about security About Touch ID advanced security technology - Apple Support

Apple doesn't really provide many answers for why they do things a certain way though.

Hi there!

There are a lot of things you can use your fingerprint for on your phone, such as paying for an app on the App Store, paying with Apple Pay, and logging in to certain Applications (such as Natwest).

There are limitations to this, as you cannot use your fingerprint to sign into online websites, or to access certain settings. You cannot completely replace your password either. The simple reason as to why is that not all Apps or websites have finger id functionality, so at certain times you will have to use your password/separate login method.

I hope this has answered your question!