You didn't state who makes the recommendation but the specification itself says no such thing. A heading begins sectioning content and there may be more than one sectioning content on a page. However, what you are asking about, is the "document outline" and it does not exist in browsers today. In particular note this reference from one of the HTML authors and ARIA:
The HTML5 Document Outline is a dangerous fiction
It is dangerous because it can lead unsuspecting developers to think
that using the nesting of heading elements in sectioning elements
actually has some effect for users who consume heading semantics.
Overwhelmingly the opposite is true. For example If you code a heading
as a h1 element and nest it 5 deep in sectioning elements, the
document outline leads us to believe that the heading will be a h6,
back in the real world the heading is a h1.
But that doesn't mean you shouldn't use multiple h1 tags. It only means you need to be careful in their usage. It does not affect SEO according to John Mueller and Google but it may affect presentation of your page in a browser. You can use this as a helpful instruction from Mozilla.
However, screen readers are a different story and first scan for heading elements and presenting multiple h1 tags may confuse the listener as to where a new section resides in the document structure. See: https://css-tricks.com/document-outline-dilemma/
For these users, the only effect of the Document Outline Algorithm was
that some new pages (eagerly adopting the new spec) were presented as
flat lists of level-one headings, with no structure at all.