by guessing, the obvious reason is that the seamless back of a jacket could provide more wind resistance as it could get relatively windy when you travel on a soundless ebike speed
another reason might be, just like i dont tend to wear slim fit cloths (like suits) for library, when arms have to reach forward for a long time, to desk or in this case the handlebar grip, surface of one’s back is slightly bigger than it normally is, keeping this position for a while arms could easily get tiresome as they have to make some effort in resisting the normal shape of cloths. i dont think many ppl would be aware of this unless arms get really uncomfortable as a result
seriously…i’m kidding, better ask them