If web programmers had the choice to install an arbitrary language's runtime in users' browsers, they would. But users have disparate browsers with disparate JS implementations, so if you want to code in a sane language with consistent behavior, you'll need to transpile and polyfill it.

You can't just distribute arbitrary binaries to users on the web, so web developers have to deal with a layer of complexity that other programs don't have to. That said, maybe WebAssembly will change this story.