Installers often get the goods from their local wholesaler at a higher price than they can find on the internet. This is justified because a lot of service is involved, which the wholesaler naturally wants to be paid for. (partly 2 delivery tours per day, orders by 10:00, delivery at 13:00, uncomplicated returns on stock items, direct contact person, etc.) If the installer only sells the material (without installation), then of course he wants to make a few euros profit for the effort, and the material is about 25-35% more expensive than the cheapest online.
If the whole thing is going to be installed, it looks different again. Although no one has to starve from the hourly wages demanded on construction sites, if money is to be made from it, it has to come from somewhere. So the profit is made with the goods turnover. The alternative would be hourly wages in the triple-digit range, as in the automotive trade, where apparently nobody cares anymore; then the material could also be sold relatively transparently in terms of price. But nobody wants that either.