Very much this. It is actually a real scandal that realtors make a share of the sale as opposed to a flat fee. The paperwork and all is the same regardless of if it's a condo or a mansion.
The time, and marketing, costs of a property tend to go up with price. If I'm buying a 12 bedroom mansion, I'm going to expect a lot more photos, better staging, and so on in the marketing compared to a 1 bed apartment.
Equally small, cheap, properties are likely to sell faster (bigger buying pool) than more expensive ones. That $20m mansion is going to take a lot more visits than the $200k apartment.
In other words, there is sufficient variability in the cost of selling to warrant a sliding fee. And generally the best way to do that is as a % of the sale. A % BTW which is negotiable based on how fast the realtor thinks the property will sell, and the sale price expected.
If you don't believe the realtor is worth 6%, then by all means skip past them. There are plenty of fixed-fee sites that guide you through the process, provide legal form docs, and so on.
Clearly not everyone needs, or wants, a realtor. But presumably there are enough who do to keep them in business. There are enough who don't to keep the "do it yourself" market in business as well.
Choice is good. What is good for you might not be good for the next person, and vice versa.
Realtors in the USA can charge 6% because they're part of a cartel. In other countries where I've lived (UK and China) real estate agents typically charge between 1% and 3%.
It's not the number of photos, as much as the quality.
Not just of the photo itself (any phone can take good pics) but in terms of how the shot is framed, how the room is styled, and so on. Basically you need a better photographer, to spend more time, and you also need to dress the room in the best way.