The current acceptable anchor element criteria state that “el is a descendant of query el’s containing block”.

This makes it impossible to use anchor positioning to attach the element to its own containing block.

Are there any technical/logical limitations that prevent us from allowing doing this?

I myself struggled sometimes with this, and I saw others stumble upon this limitation as well (for example, https://www.youtube.com/live/76hIB2L_vs4?si=qTCuTm1Js52x1Ydk&t=2191).

While it is usually possible to use the regular absolute positioning for this like with inset: 0 etc, it is not as powerful as anchor positioning itself, as we can't then use everything that it provides, like inset-area, fallbacks, etc.

In order to do something about this, authors will be required to add an extra wrapper, or stretch a pseudo-element to use instead of the containing block, both of the options being awkward and hacky.