those are product offerings, it is like saying why are there new fast-food chains if McDonald's already exists.

specifically the alternative you list are commercial offerings from other companies, that is just wanting a piece of the cake.

and for example, "internet explorer" is now going to be based on chromium, a terrible news for the web, but definitely a invented-elsewhere case.

regarding Rust a not invented here situation would be trying to develop their own rustc/cargo or a new similar language, sort of like apple with swift (not saying this is what motivated apple)