The security retention is a right according to §232 to 240 of the Construction Code or §17 para. 1, no. 2 VOB/B and is absolutely common in the construction industry. I find it highly contentious to portray this here as unusual, as this right protects the contractor against insolvency (or unwillingness) of the tradesperson during the warranty period. Of course, 5% do not heal all wounds, but it is better than nothing. During the construction phase, a 10% retention on installments is usual or a maximum of 80% a-conto payment to avoid overpayment.
Perception must be seriously distorted if one thinks tradespeople could exempt themselves from this. This has nothing to do with the construction boom but with misconception and possibly poor advice from the person who drew up the construction contract. This is, of course, how you can also train clients. "We've always done it this way," "it's a construction boom, you can forget your rights," "it has to be this expensive ...". Some just swallow everything.