Erlang/Elixir are very slow for most computationally expensive tasks.

Isn’t python the same? Most of the work is done by c++ or rust libs with python as glue.

Python is the same but there's not nearly enough incentive to move to another slow language. A lot of people that are interested in using something other than Python want a language that doesn't require everything to be written in C, C++ or Fortran and used via bindings. Having most of the ecosystem written in one expressive and relatively fast language has huge benefits.