Drug dealers in Berlin populate the parks and subway stations mostly. Easier to hide in the crowds or shrubbery, if they hide at all. Police usually never bother them.

Flip side: if you get caught, it's a lesser crime because you don't have as much on you.

Which hardly matters if your product has an excellent price/weight ratio.

My understanding (not being involved in the field in any role) is that even car-based street dealers carry far less product than one could comfortably pack on a bike.

And a cargo bike could probably move a quite respectable quantity even by wholesale dealer standards.