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.
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:somepasswordhere@example.com.
I think the arguments other commenters have made about logging, browser history storage, etc are more convincing