Because Gleam compiles to Erlang, it can seamlessly integrate with the larger BEAM ecosystem, including existing Erlang and Elixir OTP applications and libraries.

@external decorator: This escape hatch allows Gleam code to call functions defined in Erlang or Elixir modules. While it bypasses Gleam's type checks for the function call, it provides a powerful way to leverage the full capabilities of the existing OTP ecosystem.

I believe the biggest reason it's hard to replicate OTP fully and quickly is Gleam's type system (but I could be wrong).