Having commission be a percent of sales has some theoretical advantages. For example you might think the agent is motivated to get you the highest selling price. But in reality they are much more interested in making a sale at any price, because that lets them get paid and lets them move on from marketing your property.
Residential real-estate transactions with a mortgage are about as regulated and standardized as it gets. The listing agent actually does very little beyond getting the property listed in the MLS. The difference in the amount of work they do in selling a $100K house vs. a $900K house is small (in fact the $100K house might take a lot more work because at that price it's probably got some serious drawbacks), so why should the higher sale price pay them much more?