It's just realism.

MacOS is unix under the hood so the models can just use bash and cli tools easily instead of dealing with WSL or Powershell.

MacOS has built-in sandboxing at a better level than Windows (afaik the Codex App is delayed for Windows due to sandboxing complexities)

Also the vast majority of devs use MacBooks unless they work for Microsoft or are in a company where the vast majority of employees are locked to Windows for some reason (usually software related).