True, but for injection moulded parts you need to have a large enough production volume to justify the tooling costs of a new mould and hassle/costs of stocking and managing yet another line item. Having most unicycles using the same parts helps to keep (even increasing) prices down.
I think we are increasingly moving to a point where there are distinct products for beginners as well as more advanced riders though.
Beginners are more likely to appreciate more resilient and protective bumpers and more curved saddles with plenty of padding and wider front sections, whereas more advanced riders tend to lean towards flatter bases, less padding, and less width at the front.
Personally I’ve no interest in a larger bumper for my KH Fusion One as an example, partially because it would need to be so huge to actually protect anything on my 36" wheel, but also because I can’t remember the last time I scuffed the back of the saddle on the ground. Handlebars at the front yes, anything on the rear no.