Why is _no one allowed_ to install my washbasins?

Get tile drill bits and practice on an old tile.

...The only way in my eyes, 10-100€ under the table or a friend for free.

I could quickly drill 15 shelves into the bathroom wall in the rental house. The landlord will surely be thrilled.

You’re not seriously recommending illegal work, are you? Are you kidding me?

So the majority thinks they just don’t want to and still say "aren’t allowed"?
I find that strange. For example, the carpenter, he’s in the house anyway because of the stairs. He wouldn’t even have travel costs, he could just give me any nice invoice for it. :/

The plumber also said something about warranty claims, but I didn’t fully understand it. He’s also in our house. He installs the tub, the toilets, and the shower. I said we could also exclude the warranty. But he said "no matter what I sign," if it went to court, he would always be liable if water comes out of the wall somewhere.
But I wonder if he wouldn’t be liable anyway if water comes out somewhere that’s not a pipe. I mean, it can’t be my washbasin if, for example, water flows into the wall under the plaster?
If water comes out of the pipe somewhere and runs into my bathroom, then it’s my fault / the fault of the washbasin installation. That can be clearly distinguished, right, or am I mistaken?