Apple likes to present the AppStore as the only thing protecting its users from the Wild West.

Admitting their sandbox could be turned on by default and give the same protection without having to go through their vetting system and giving them their cut would be counterproductive. How would they justify it makes sense on the phones and iPads then?

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.