I've been wondering about that for a while... iOS was a newly created platform, and I suppose Apple had the right to impose any restrictions on it that they wanted, but "freestanding" extensions have been permissible for years, and any attempt by Apple to totally restrict their content for any reason could be considered censorship, perhaps even monopolistic. I certainly hope they'll continue to allow user discretion as they have in the past.
A major rationale for App Store exclusivity on iOS has been to help ensure stability, reliability, and security of iOS devices particularly in light of the fact iOS has less protection from ill behaving third party software. As far as I know there has been no indication of Apple's adopting that model for MacOS. Instead Apple relies on ease of use and automatic updates to encourage users to go first to the App Store for their needs and billions of dollars in sales to attract third party developers. That strategy has certainly worked on me.
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If you think about it, Safari extensions are more akin to iOS apps than MacOS apps. Apple has a vested interest in assuring the stability, reliability, and security of their flagship browser and that is a lot easier to do when they can control new extensions through the App Store mechanism. It hasn’t been that long ago when you and others were questioning the functionality of allowing or disallowing content blockers on a site-by-site basis in Safari. It turns put the feature works well
IF the content blocker developers used a new API to connect to Safari, but many developers continued to use the older deprecated API. Of course the blame fell on Apple not on the third party developers when the feature did not work. The App Store model allows Apple to control this sort of thing as well as watching out for unintended (or intended) vulnerabilities to mallicious operators.
