When I did freelance all big US companies forced me to go through an “approved vendor”. Most of them offered only W2 and would take a huge 30% or more cut. In the end I found one vendor that only took a 3% cut and otherwise left me alone. I think for the big companies it just makes accounting easier if they go through less vendors.

From my experience in DE, it depends on the company size. Larger enterprises probably will go through an agency, probably to (try to) cover themselves against accusations of false self-employment (Scheinselbstständigkeit).

Anecdotally, the two contracts I had through agencies were still the most lucrative financially, despite the middleman taking a hefty cut, simply because large enterprise pay higher rates.

Even in DE, there are issues around "Scheinselbstständigkeit" if you don't have a UG or something. Most companies prefer to work with other companies, even if those have just one employee.