I’m with you, despite seeing lemmings downvote the heck out of your comment 😢

The reason, and specifically for whitespace at the beginning or end of a password, is that a lot of users copy-paste their passwords into the form, and for various reasons, whitespace can get pasted in, causing an invalid match. No bueno.

Source: I’m a web developer who has seen this enough times that we had to implement a whitespace-trim validation for both setting & entering passwords.