So why not just use JavaScript to begin with?
If you're a Ruby programmer, you don't need to learn a new language, and you can have confidence that the Opal guys have looked after a lot of the issues you're not experienced enough to understand.
Your question is like asking a C programmer "why not just use assembler?" or an assembly programmer "why not just toggle 1s and 0s in, because ultimately your code is going to get turned into binary eventually, right?"
Abstractions and different models of describing a problem solutions are helpful for a multitude of reasons.
In this particular case, Ruby is not being compiled to JavaScript but being transpired.
First, "transpiled" not "transpired".
Second, "transpiled" is a special case of "compiled", so you can never correctly say that something is not being compiled but is being transpiled, in the same way you can't say something is not a rectangle but is a square.