There's probably enough of a difference where it wouldn't hold up in court? Or it could be opening a can of worms.
Proprietary things like logos, names, designs, etc, are far easier to go against where as a structural design that's been around forever and is all pretty similar through and through wouldn't.
I mean, do you want each company to start being able to copyright every single detail where progress can become stagnant? Or is having each company be able to draw ideas from each other to try and one up each other better?