Someone just leaving comments and not signing off on reviews isn't helping unblock anyone and should put in more effort to be willing to sign off and move the work forward. If the most people in the org thought this way nothing would be committed and everyone would have 'non-blocking' comments to deal with.
Another way to look at this is in absence of another code reviewer, not signing off after commenting is equivalent to passively blocking the PR and can be a bit toxic depending on the circumstance.
I'm probably missing a scenario (maybe there's a bunch of people you know will review the code for instance) that this makes sense so happy to learn where/when specifically it makes sense :)