I'm not asking about the usefulness of the languages that do exist already: I already know, and agree, that different languages are better for different purposes.

However, why don't they just have a single language that can do it all? Why, when C# was created, they didn't keep everything from C and C++ and just add a few things, so that it can be used as both a low-level and high-level language? I see no harm in adding all kinds of commands to a single language that would allow it to be good for everything, and even eliminate the need for all other languages.

Someone from another thread said that if there's a flaw in a certain language, its successor might not have it. However, why don't we just update that language to remove the flaw, and/or add anything that's missing? Arrays are different in Java and C#, but why not have them both, just use different commands for them? And so on...