Why does the Terraform.azurerm provider handle registering Microsoft.providers

Terraform Providers Azure
1

Just kind of started wondering what the background on the design as to why when I define an azrurerm provider block, that it handles the registration of Microsoft’s (feature) providers ?

I saw the new options regarding the registration of Microsoft providers in v4 and it made me want to ask why are providers handled in the Terraform provider and not azurerm_subscription ?

Why

provider "azurerm" {
  resource_provider_registrations = "core"

  resource_providers_to_register = [
    "Microsoft.ContainerService",
    "Microsoft.KeyVault",
  ]
}

and not

resource "azurerm_subscription" "example" {
  subscription_name = "My Example EA Subscription"
  billing_scope_id  = data.azurerm_billing_enrollment_account_scope.example.id
  
  resource_providers_to_register = [
    "Microsoft.ContainerService",
    "Microsoft.KeyVault",
  ]
}