Why can’t use the page slug 1 ?

I’ve tried to name a page with 1 and expecting the slug to follow accordingly. Except, it keeps reverting to 1-2

I’ve tested this on a clean install and other sites I’ve got. But all of it doesn’t allow me to set the slug for a page as 1. And none of those have pages, posts or media labeled with 1.

Strangely enough, I can set a post with a slug as 1 but not a page.

I’ve tested this on 5.9.3 and 6.0 – similar results.

Is there something amiss with the core?