That's how the site was when I started it. Had a section for Cat, Westies, Birkins, Ultralite, Stalker, Rotus, etc... Ran it that way for a while.
There simply aren't enough sevens to support that structure. You end up with a bunch of sections with MUCH less traffic than what we have in the tech section today.
So I consolidated them all - later on, I had a few requests for a bike engined section, so I created it. When the demand is there, I will change the structure.
Also, among Birkin, Westies and Cats (majority of the sevens), their commonality is as much based on their drivetrain choice, than brand.