[amanda99]

Yes, people share usernames and passwords, but there's no single canonical string, like "username=amanda99&password=hithere". For example most of the time when I share user/pass combos, they are in separate messages on Signal. You type them into two different boxes, so you normally copy the username, then the password in separate actions.

[nightpool]

I mean, for HTTP Basic there literally is a single canonical string, and it's not uncommon to see people send you links like https://user:[e-mail].

I think the arguments other commenters have made about logging, browser history storage, etc are more convincing