I've wondered about this too. It happens in commercial, but isn't as obvious. My best theory these days is that we are asked to model behavior of the typical buyer in the market to develop an opinion that is frequently a point value. Accuracy and intellectual rigor in our world as appraisers is required.
Brokers are in a different world, even though its the same real estate. They aren't modelling the typical buyer behavior. They are trying to find it, capture it, and bring it to the closing table. Their accuracy and rigor show up in the negotiation process which concludes after the last argument after the closing. The exaggeration creates the interest which creates the offer and so on.
Not saying its fair or even right, but that's the theory I'm testing these days.
My limited dealings with commercial brokers is that they were more professional and fact based .
I used do sell real estate ( for over 5 years ) prior to becoming an appraiser, and on a personal level, I can enjoy interacting with agents. But as far as business, I have a low level of respect for the typical residential RE agent. They do have to follow the law especially regarding fair housing, contracts, escrow funds (if a broker ), and they by law must disclose known defects. Other than that, pretty much anything goes.
As you note, even though we are handling the same product, they are in a different world. Their world is the sales world Their time is spent marketing themselves, getting listings, and snagging buyers. Though they like to call themselves "market experts", most are not , (with a few rare exceptions who I do respect) .
Appraisers should also consider that the average RE agent farms and works in a very small area and type of property ( again, with exceptions ). A RE agent might specialize in luxury beach area condos, or tract homes in her own subdivision. The agent rarely sees, lists, sells, or knows anything about other properties or areas beyond their niche - even though, like anyone else, they can pull data at a click, and then proclaim they know all about the entire region.
The other thing to consider is RE agents are under no obligation to list sell, or negotiate to a benchmark of "market value". They have no interest in MV for its own sake, except that in a financed deal, they hope the property "will appraises out". Their goal is sell the property, make a commission, and whenever possible get the highest price so they can use that as to dangle as bait to get more listings..