Why? We can know that, in fact, taught about it in a class I developed. But I've reviewed a lot of appraisals from CRs and few address anything but accrued depreciation in either the comps or the subject. I reviewed one report where the appraiser obviously adjusted the cost approach to match the sales approach and to do so, without addressing depreciation, simply stated the RCN was $50/SF at a time when construction costs were north of $80. She literally had zero depreciation of any kind. And no consideration of depreciation of comps or the subject. You cannot do one without the other. CGs are more likely to address the externalities and functional issues but again, they still tend to address only accrued depreciation rather than explicitly address both external and functional depreciations separately.
In summary, this is the Donald Epley method of extraction.