Earlier quoted context omitted.
Why would they not allow you to sandbox non-play store apps?
Everyone wants their phone to be just as open as their computer and without restrictions - you have that with Android. The minute apps are forced to be in a sandbox, you forgo “openness” and “choice”.
QubesOS, Snaps, Firejail, Sandboxie, browser tabs to some extent, restrict user-freedom in only the most technical way, with no bundled choice traps. On the contrary, a good sandbox allows users to forego human curation/proprietary malware detection services and execute any code they want.