I wonder if it's down to having an opportunity to negotiate the price. It's either that, or things like handling get calculated to fine points - a single screw being more expensive than a box of screws, for example. Shipping is complicated as well. Maybe it's about setting up invoicing accounts as well?

Anyway, lots of barriers, makes you think it's either a requirement that I as a layperson don't understand, or a hole in the market.

That said, what about German electronics parts sites like Conrad or Reichelt?