There are a couple of problems with the argument you're making:

1. Any app can be sandboxed, not just Mac App Store apps (the only link is that Mac App Store apps require sandboxing).

2. Enforcing sandboxing on macOS would hinder industries Mac users value, per my comment here https://news.ycombinator.com/item?id=44952088 Apple would love to enforce sandboxing by default, because it would serve their long-term strategic goals (moving computing towards devices that benefit from integrated software/hardware), but it hurts their short-term goals (maintaining Apple's [somewhat tenuous these days] penetration across a variety of particularly creative industries) too much to do so.