Why is _no one allowed_ to install my washbasins?

Haha, "would be" would be subjunctive *LOL* But let's turn the tables and naively ask the builders: where is the GU supposed to get his profit from – does anyone seriously believe that would work with the item "stone stacking"?

well, at €450,000 his profit shouldn't necessarily fail because of his undeserved €4,000 sanitary equipment

besides stone stacking, there are 100,000 other items where the GU earns tremendously. that's what he's there for, at least for me

not that. But in total, it makes quite a difference. Depending on how many projects your [GU] has in a year.

Well, there are still possibilities. Either agree on just the rough installation (with reference to your own selection), or negotiate the price down. In doing so, under no circumstances should E-Installation be forgotten. Otherwise, you will also pay 120€ net per socket.

Under no circumstances should the electrical installation be forgotten. Otherwise, you will also pay €120 net per socket.

That's right. It's best to have 100 sockets, n outlets, and m network sockets included in the contract right away.

We partly procured washbasins and light switches ourselves because the service provider could not order these themselves. This was not a problem in the single contract award, and the service provider certainly earned well from us. And rightly so, as they also did good work. What does not work well is the attempt to reduce the price through material purchases by the service provider. No one will be happy with that.

To put it differently:
Can you buy brake discs and calipers online, bring them to the authorized workshop, and only pay the labor cost for installation?

Yes, that’s possible nowadays... also when it comes to car tires.

This industry is simply a cartel or a mafia

Unfortunately, that is the case in some places. With a good plumber/GC, a solution can probably be found that works for both sides. I absolutely understand margin & warranty costs, but less so the "mafia & cartel". But there is also something in between.

This is a much-discussed topic and you simply have to see what is possible with your GC. Of course, he needs a margin and his warranty must also be properly paid for. In our circle of acquaintances we had the case that, for example, they had sum X available per bathroom and could shop as they liked.
But as already writes, there are some mafiosi, and with such a person I would take a closer look. As is known and also occasionally read here, there are different behaviors on the market.
I think there is something in between, namely something that is far from "ripping off customers" and just as far from "disrespectful behavior towards craftsmen."