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 mentio

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.