I have a friend who managed a RE brokerage locally and she told me many agents did BPO's for this reason - the fee stank, but it gave them an opportunity to market to the properties around it they just did a BPO on a neighbor, they are the market experts, they'd be happy to drop by and give a CMA etc - end goal is to procure listings or buyers
Inserting them into the appraisal elevates the status of the RE agents or REALTORS, and weakens status of appraisers. In the eyes of an owner or buyer, it just looks like the appraiser was too lazy to inspect or the RE agent is "better" at it, so that is why they are there instead.
This debacle is a failure of USPAP. USPAP does not require an inspection, but it could make inspecting a property for appraisal use/purpose part of appraisal practice. Instead, inspections are treated as a grey area, if they involve an opinion they are considered part of appraisal practice, if no opinion then not.
Note that the appraiser will not be having interaction or contact with the third party inspector, lest the conversation bring forth an opinion from the inspector -