This is just a guess, but I’d think that more expensive houses have more room for changes to be worth it to increase the value. If you have a $200K house, the increment you’d get from carpeting or painting probably isn’t going to make you any money. If you have a $2 million house, which might not be much bigger than the $200K one, the cost of the upgrade won’t be much more, but the upside could be a lot more.
Good advice from a REA could really pay off for the more expensive house.
Plus I assume there’s more room for handwaving, less room for comparison between subdivision housing and big mansions. My house is valued at above $500,000 but sits on a 50-foot-wide lot and there’s probably 4 houses on the block with the same/similar floor plan and comparable amenities. (My neighbour is the same but flipped, and has a 3-car garage instead of two, etc. etc.) What the neighbour’s sold for is a good indication what mine is worth. What other houses sold for, square footage is more easily compared as they are all pretty much the same build.
With giant mansions on large lots, possibly with prominent views, pricing is probably far more arbitrary. Extras like security features are probably more important. I recall some article about a mansion listing for $40M and dropping to below $20M before it sold. I can’t imagine that sort of wiggle room in normal middle class housing prices unless the house was a former grow op, flood damaged, or something.
And as mentioned, there’s not a long line of people looking to drop $20M on a big mansion; the list of prospective clients is far smaller, meaning more footwork is needed to find and convince a buyer.
It might be a pain to buy the home of an NBAer. I can barely reach the top shelves of the cupboards I have. 
When I say $2 million house, I’m talking about subdivision housing. My subdivision is fairly old (for California), about 70 years, and all the houses are $1.4 million - $2 million. There are usually 4 or 5 for sale at any time - or in the sale cycle, since they mostly sell in under a week these days.
$20 million dollar houses might be different, but there are towns in Silicon Valley with tons of these. I don’t know how many are in the market at any time.
Yes, it’s hard to have a discussion about “low priced vs. high priced” homes when by normal human standards, pretty much all housing is expensive in quite a few areas of our countries now. I doubt there’s many single family dwellings in Toronto that sell for under $1M today.
In the last few years the push has been for non-commission real estate. In a seller’s market where the homes sell themselves, many people would rather pay a flat fee to have the house listed and sales paperwork handled, than pay a percentage commission that seems excessively high.
Not sure how this sort of thing is received in California. I do recall hearing of several of these flat-fee agencies going to court across North America over the exclusion rules of the MLS system, percentage buyer’s agents refusing to show flat-fee listing houses, etc.
I haven’t seen any ads for flat fee places. That doesn’t mean that there aren’t any, but we get three or four pieces of real estate mail a week. I have seen ads for fly by night “we’ll buy your home for cash” places.
The thing is that the cost to upgrade a house, cosmetically at least, has stayed reasonably stable while the cost of a home has soared. That makes upgrading a better idea in high price areas.
“Purple Bricks” is a company that I gather started in Britain, and is now actively advertising in Canada. Not sure if they’ve hit the US market. I think they bought ComFree which was the earlier flat-fee house seller. (Basically, they handled the paperwork and listing - the rest is up to you, i think).