Because you usually don't have control over that. On lots of system, any application that can display an UI can access the current clipboard's contents.
Taking a step back from the problem, users do usually have some amount of control over what is running on a system.
I mean, I understand that computers managed by typical users will be a shitshow, but much of the problem in that situation is that they don't really care.
I believe the browser is one of the few applications that already take this into account, fortunately. Clipboard access from Javascript is secure, for example.
I mean, I understand that computers managed by typical users will be a shitshow, but much of the problem in that situation is that they don't really care.