This topic comes up periodically. Here are a few links to previous discussions where IDers have written about why they don’t always include comments when disagreeing:
https://forum.inaturalist.org/t/dissenting-id-comments-would-be-useful/34178/2
https://forum.inaturalist.org/t/when-a-user-disagrees-this-is-a-certain-species/25825
https://forum.inaturalist.org/t/require-a-comment-when-adding-a-coarser-id-to-an-observation/45736
https://forum.inaturalist.org/t/comments-made-when-suggesting-alternative-identifications-or-corrections/13823
The short answer is: in an ideal world, yes, people would include comments when they disagree with an ID. But time is short and there aren’t enough IDers for most taxa, so people often prioritize IDing over typing comments for observers who may not read or care about the comment anyway.
If you want to know more about why someone made an ID, it is always OK to ask – most IDers are happy to provide an explanation if the user expresses an interest.