I think you're assuming way more thoroughness in the typical assessment of a property than is warranted by the practices. At least with respect to real estate taxes, in my experience assessments are pretty mechanical with respect to square footage and standard house features (number of bathrooms, etc.)

Assessments are different from inspections.

I don't really disagree with you though. There would seem to be a difference between someone being directly paid to assess something, perhaps largely based on standard formulas and practices and a web company providing housing estimates with no contractual relationship.