I guess you have never seen a Stromer[1]. The iPhone of e-bikes.
They weight near nothing for power levels legal on bicycles.
Even on low end mopeds, motor weight is not that big of a problem.
It is at power levels of around 7.5kw, when it becomes a problem.
So I never considered buying a mid-motor ebike.
Having it apply torque via the chain/belt also mean you can gear the motor output on your back wheel.