A given programming language often has limitations which are largely different than the limitations from others. This means that different languages are often used on different kinds of problems. Want something fast, use C. Want to write something quickly, use python. Want it to run on just about anything, use Java. And so on.

So why don't we make one ultimate one or a few that fulfill all needs? Well, partially because we haven't figured out how to do that, but also it's really easy to learn yet another language once your understand how they work. I can write in python, js, c, c++, c#, Java, kotlin, rust, perl, ruby, php, forth, lisp, and I could keep on going for quite a while. The underlying concepts are largely the same and so picking up a new language is no big deal (though being good at it is a bigger deal). We have so many because ultimately it just doesn't really matter that we have so many.