Partly because sometimes a particular language suits a particular problem set.

Partly because people just like writing computer languages.

But mostly because people mistake the fundamental problem of programming, which is programming is really hard. So someone comes along and thinks “Programming is really hard, it must be a problem with the languages available” and sets out to write a computer language that makes programming easy.

But all that happens is they trade one set of difficulties for another set of difficulties. They might succeed in making writing the initial version easier, but make maintaining that code harder. Or they might solve some memory allocation problems, but create performance issues.

Either way, someone will write a language because they think they will help solve the issue of programming being hard, and fail. Because the really hard bit about programming is about understanding everything the program needs to do, in microscopic detail, and translating that into a structure that best fits the problem; not the actual coding itself.