Well there are various reasons…

Browser updates don’t always work as expected. When you are doing a presentation, I strongly encourage to disable your updates. I’ve seen it more then once when something suddenly doesn’t work. Not possible? Chrome plans remove alert(). Firefox and Chrome disabled TLS 1.0 quite recently (and obviously some sites were not ready, some still aren’t).
When you have many tabs this not a 30 second restart. My wife has literally more then 100 tabs open. Restarts are slow as you can imagine (she uses Chrome).
You might loose your work. When you have many open tabs you might forget to save. And webpages don’t always save drafts or warn before closing tabs.
You loose your sessions. Tabs can be restored but session cookies are removed (HTTP sessions are removed too). You might need to re-authenticate to various sites after restart.
Hibernation. Some people just hibernate their computers. I do. That way everything is ready when you need it. Even if I don’t have that many tabs I can have a browser open for months.

So yes, unfortunately this is not as easy as it seems.

The macOS limitation on evergreen generally applies on iOS/iPadOS (especially since all browsers on iDevices are Safari).

On Android, many users disable the Google Play Store from auto-updating apps. After being burned when their favorite apps munge the UI or kill features, they are more cautious. These users are not updating their browser as much as a result.

Gathering anecdata from a half dozen Android users I know, their Chrome and Firefox versions averaged 10 versions behind.

On my own site, when I look at just browser versions and then compare mobile versus not, the older versions (70s and 80s) tend to more prevalent on mobile (with the 30s on desktop, perhaps owing to web views).

There are standalone updates for Safari but they are only available for the previous two major versions of macOS. For instance, Safari 13 (released with macOS 10.15) is only available for macOS 10.14 and 10.13. (The Safari 15 release notes don’t specifically say, but according to Wikipedia it is not available for macOS 10.14 or older.)

Cuz Safari is the new IE?

Worth nothing, as a nuance, is that you don’t need to perform the macOS update in order to get the Safari update. This is also reflected in your screenshot: you can uncheck the macOS update checkbox. Because of this decoupling, older macOS versions (which might no longer be updated) can also receive Safari updates nowadays. This wasn’t always the case though, hence the confusion.

On iOS it’s an entirely different story, unfortunately. AFAIK, there, the Safari version updates are still tightly coupled with the iOS version updates.

And yet.

What s***y workflow do they have if a 30 second restart disrupts their daily workflow? Don’t they take breaks? If they are not monitoring a nuclear reactor, this explanation doesn’t make any sense.