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Really the idea of "backend" and "frontend" is a web phenomenon that does not really have the same equivalency in other spaces. MANY of the languages you mention were not developed for use in the web space and have had libraries built to support their use in that space: C++, Rust, Python, Java, Ruby (?) all fall into this camp. Yes, they can be used for web stuff, but it is not their primary purpose (unlike PHP, JS/Node, etc). As a result I do not find this counterpoint to have a lot of meaning in the context of the question. Though, why are there so many programming languages is an interesting question in its own right.

I think it's less about requirements driving the existence of languages and more about the audience - C# as a clear example of Microsoft ensuring its developers are upgrading to a technology that favors them instead of switching to Java.

Not to say requirements don't play a part - Python exists because of Math/prototyping requirements of people that don't want to worry about memory or complex syntax (generics, etc..)

We shouldn't compare creation of languages to frameworks. If you look at back end frameworks, like web MVCs or ORMs, each language usually only has one or two major players.