There are (at least) two forces pulling in different directions: I can envision common form factors for mid motors and even motor/gearbox combos. That's most of the parts compatibility struggle. OTOH e-bikes can have a very wide range of designs and still be practical for all purposes. You don't have to optimize for weight. Your cargo ebike can be your commuting bike.
Also, there are niggles like belt drive requiring a different frame. You have to be able to put the belt through a gap in the... do we still call it a "chainstay?"
I've been shopping for a used bike. Life was so much simpler when I had no idea that Shimano and Campagnolo drietrain components were so incompatible and that incompatibility flows through to the way the rear wheel is built.
Also, there are niggles like belt drive requiring a different frame. You have to be able to put the belt through a gap in the... do we still call it a "chainstay?"
I've been shopping for a used bike. Life was so much simpler when I had no idea that Shimano and Campagnolo drietrain components were so incompatible and that incompatibility flows through to the way the rear wheel is built.