Because it takes a long time. On the rare occasion I log into something on my phone, it takes almost a minute to type in the random passwords that Keepassxc on my computer generates.

Fingerprint authentication will work though.

Once you're logged in, just copy/paste the password? You only need to type out your master password manually.

Am I misunderstanding the issue?

On Android you should not copy/paste as all apps can read the clipboard. Use either a keyboard that comes with the password manager, or system auto-fill support that gets the values from the manager. Both modes are supported at least by Keepass2Android.