They often do. Panasonic make a superb very high torque rear hub motor which some German and Austrian makers have fitted, and they have also made a range of average power front hub motors, including one that is also an electronic brake.If hub motors are so good why are respected big-named manufacturers such as Bosch, Yamaha, Panasonic and Shimano producing crank motor systems at all? Why aren't they making hub motors?
The previous Shimano STEPS system was a hub motor one.
Fashion is an important element. Having been around e-bikes for decades I've seen it swing between crank and hub motors more than once. Currently it's a crank drive period, but in the mid 2000s they all but disappeared with hub motors dominating.
.