I thought they build turnkey houses no matter on which plot of land.
No, many prospective builders mistakenly think that way; "Bauträger" would be the correct technical term for a contractor who handles the entire construction turnkey from a single source. But that is not the case. The correct term is "Generalunternehmer," aka "GU." A "Bauträger" often also acts as a GU – but he is only called a "Bauträger" if he is the owner-builder who develops the land himself and then sells it in its fully developed state. Many interested homebuyers don’t understand this – probably because recently 1. the purchase agreement for the land with the house already on it is often concluded before construction begins, and 2. the Bauträger grants the buyer extensive opportunities for design input, which the buyer otherwise only has if he is the owner-builder himself. If the house buyer already owns the land, he can no longer hire a Bauträger – but he can engage the same company, with which he then concludes a construction contract (i.e., a contract for work and services, not a purchase contract). This is mentally too complicated for 99% of laypeople – probably because it is a complex matter.