Security. App store has much tighter rules on what a developer can do whereas downloading a dmg from somewhere pretty much leaves you to fend for yourself (and Zoom already has proven itself here).

Edit: clarifications

Because at this point, every single version of the app from their site has had some issue that wouldn't have happened in a sandboxed Mac App Store version.

Because the non-app store version does weird stuff - faking installation, background services, etc. Sandboxed version would hopefully be less malware-like.

No, it's probably because then the app would need to be sandboxed and they wouldn't be able to justify their insane Hardened Runtime exemptions.

Since they are already on iOS, this is not an excuse. I would only trust a sandboxed version from them.

Apple tax on free?

Their macOS app is already notarized by Apple, there is no additional "Apple tax" for them to make a Mac App Store version.