Because this often makes the input feel broken to the user.
Instead of asserting that "users are too stupid" as you have done earlier, perhaps programmers should be less stupid and write more permissive parsers.
I'm not using "stupid" in the derogatory sense. I'm using it as a recognition of the skill/knowledge gap between technical and non-technical users.
To clarify, we get _asked_ by our users to implement fields that limit input to help them avoid mistakes. Our QA and UX teams agree. This isn't a unilateral engineering decision.
Yes, you want to provide guidance, but without getting in the way.
That's why I was suggesting letting the user paste in these 'wrong' characters, but offering help in removing them.
Of course, that's a trade-off. And it's more annoying to design and implement than just forbidding those characters from entering the input field in the first place.
Instead of asserting that "users are too stupid" as you have done earlier, perhaps programmers should be less stupid and write more permissive parsers.
Accessible design is actually pretty hard, and most designers and programmers get this wrong. If you want some good design advice, you can start here: https://adamsilver.io/blog/the-problem-with-live-validation-...