There is, it's called WebAssembly. But while supported by major browsers, it is still growing, not being widely available a year ago. It still lacks features making it mostly useful for heavy computation (which is its strength anyway).
WebAssembly is meant as a compile-target, which means that programmers would usually write code in for example Rust, C++, Kotlin, ... which would be produce WebAssembly code to be run in the browser.
That way, many languages can be used, while browsers only need to support two. It'd be an impossible task for browsers to support all the languages that exist natively, as there are thousands.
Javascript is also used in this role (often with 'extension' languages like CoffeeScript or Typescript, but also different ones like Kotlin). But it's less suitable in that role because it was designed as a high-level language (and is overall not very well-designed anyway). That makes it slower, and awkward to compile to from lower-level languages (usually compiling is from higher to lower level).
So, there are starting to be alternatives. Browsers themselves will support two languages, one of which is meant to be generated by compiling other languages. But this will make it more viable for developers to use any language that supports WebAssembly as a target, which will probably be most of the big ones in the future.