It's my understanding that this HTML is (mis)used to reduce tap-delay on mobile devices:

https://stackoverflow.com/questions/6397748/whats-the-point-...

That's doesn't excuse it, of course.

I think it's boilerplate that ends up pasted into a lot of websites.

I have no clue why people downvote you, as this is exactly the reason this page is not zoomable. In my humble opinion people doing stuff like this should be forcefully separated from the process of creating web sites.

There’s reasons for doing it, though. For example, iOS Safari likes to zoom past 1.0 slightly when focusing an input and doesn’t zoom back out to 1.0 when unfocused. This often leaves users in a slightly zoomed in state which means they can easily miss right side aligned buttons and content. Setting the maximum scale to 1 in the viewport prevents this.