You are trying to construct a scenario, where you have the ability to elevate your own rights to somebody else's repo's contents. Name a computer system that intentionally allows people to do that. That similar to demanding that you can still send emails on a terminated email account from your prior employer.

If you want to continue your access to that private repo's source code, you now need to speak to them. They own it, not you.

I am not trying to construct anything. I just described what happens and what I expect.

"where you have the ability to elevate your own rights to somebody else's repo's contents."

This is not an accurate description. There are two repositories: the original repository, and the fork. Nowhere I want to to elevate my rights regarding the fork to the original repository.

" Name a computer system that intentionally allows people to do that."

If somebody sends me a word document per Email, I can edit it without someone else being able to delete the modified word document.

You keep trying to claim that a different person's private repo that you have forked has somehow given you ownership over the contained information. That's just not the case. Forking their code doesn't make it yours.

actually that is the point of a fork onto my account

GitHub doesn't allow you to make a public fork of a private repo, either. When you make a fork of a private repo, the resulting repo is constrained to have no broader access than the original.

"GitHub doesn't allow you to make a public fork of a private repo, either. "

where did I claim something like that?

I'm saying that GitHub consistently does not allow you to control access to a private fork. The original owners retain control.

how does this consistency manifest itself?

what about when the original owner publishes his repository? he doesn't control the visibility of the fork, does he?