Because once you introduce threads, your program complexity increases massively. You're now throwing a few groups of instructions at the CPU and saying "Hay! Execute these instructions in any order you like". Then you're adding code to deal with the issues of that random execution entails.

Also, it's not faster, so what's the actual point?

The browser should use all CPU cores efficiently to make javascript fast. Exposing WebWorkers to javascript programmers IMHO is totally the wrong way to do that.