Why can't to-infinitive be used as subject in "Not to learn French is my biggest regret."?

Re:

• Not learning French is my biggest regret.

but not

• *Not to learn French is my biggest regret.

Really, the negative is just a distraction. You can't say

• *To learn French is my biggest regret

either.

The reason is simply that the complex predicate adjective be one's biggest regret does not allow an infinitive subject complement. If if did, it might be extraposed, but, no:

• *It is my biggest regret (not) to learn French

At its most basic without a full lecture:

Gerunds are used when actions are real, fixed, or completed. "I enjoy cooking."

Infinitives are used when actions are unreal, abstract, or future: "He wants to swim."

...........but there are loads of exceptions!