why breakpoint in `Project.application` function of `mix.exs` is useless？

In mix.exs file of vscode, I set breakpoint in line of mod: {KV.Application, []}, # breakpoint here.

When starting debugging, it has been ignored. Why?

defmodule KV.MixProject do
  use Mix.Project
  #use Application

  def project do
    [
      app: :kv,
      version: "0.1.0",
      elixir: "~> 1.13",
      start_permanent: Mix.env() == :prod,
      deps: deps()
    ]
  end

  # Run "mix help compile.app" to learn about applications.
  def application do

    [
      # mod：{KV,[]},
      # mod: {KV, []},
      mod: {KV.Application, []},  # breakpoint here
      extra_applications: [:logger]
    ]
  end

  # Run "mix help deps" to learn about dependencies.
  defp deps do
    [
      # {:dep_from_hexpm, "~> 0.3.0"},
      # {:dep_from_git, git: "https://github.com/elixir-lang/my_dep.git", tag: "0.1.0"}
    ]
  end
end