First of all, depending on the size, manzanita branches usually go for 1-2$ each on the hobby market. And no, manzanita looks nothing like dead oak branches. For the most part, manzanita comes in a greater variety of more attractive shapes and sizes which makes it more desirable, along with being rot resistant.
Another thing about manzanita is that it grows pretty much exclusively along the west coast, mostly in the pacific northwest. So while I could go on a 20 minute walk and come back with a full hardscape, somebody in, say, west virginia doesn't really have that luxury. Not to mention people who don't have time or access to go collecting.
Wabi Kusa is just an Aquascape Katamari