Because there are not a lot of high quality examples of code edition on the training corpora other than maybe version control diffs.

Because editing/removing code requires that the model output tokens for tools calls to be intercepted by the coding agent.

Responses like the example below are not emergent behavior, they REQUIRE fine-tuning. Period.

  I need to fix this null pointer issue in the auth module.
  <|tool_call|>
  {"id": "call_abc123", "type": "function", "function": {"name": "edit_file",     "arguments": "{"path": "src/auth.py", "start_line": 12, "end_line": 14, "replacement": "def authenticate(user):\n    if user is None:\n        return   False\n    return verify(user.token)"}"}}
  <|end_tool_call|>