It's trivial to get something past app review (eg. look at casino apps that were disguised as games)
On the Mac, staying on the Mac App Store makes sense because it is the easiest way to enforce you only install sandboxed apps.
On iOS, that's not going to be necessary, because every app on iOS is sandboxed.
As an aside, I do believe you can use sandboxd with apps outside of the App Store, someone just has to write security policies for them.
This page suggests that Apple might want to deprecate use of sandboxd in favor of just the App Store[1], though.
[1] https://codeberg.org/valpackett/rusty-sandbox
Yes, you can, it's trivial to do so. However, Apple does not make it easy to find out if an app is sandboxed or not, or what permissions it has.
The warning that macOS shows when opening an app not from the Mac App Store does not differentiate between sandboxed and non-sandboxed apps.
It's trivial to get something past app review (eg. look at casino apps that were disguised as games)
On the Mac, staying on the Mac App Store makes sense because it is the easiest way to enforce you only install sandboxed apps.
On iOS, that's not going to be necessary, because every app on iOS is sandboxed.