I think the fees charged are simple economics. In the UK the property market is more active and the average prices are higher. A small percentage of a high turnover generates as much money as a large percentage of a small turnover. French agents would not be able to stay in business if the fees were set at the level found in the U.K., the business would not be profitable.
I also disagree with the idea that there is no incentive for the vendors to shop around for the lowest fees. I think there is. When people are selling a house they know how much they hope to walk away with. If for example they ‘want €100,000 and have the choice of two agents, one that charges 5% and the other 10%. To get their €100,000 through the first agent the house would be marketed at €105,000, with the second €110,000. Buyers would find the second agent more attractive and the house would probably get more viewings. Buyers don’t go looking for a €100,000 plus fees property and look at properties with total costs within their price range.