It seems obvious. If i spend 600€ on a new bike and 600€ on a conversion kit, i will get a very high spec e-bike. But i would have to spend double that, about 3000€ to get an okay quality off-the-shelf e-bike.
The off-the-shelf e-bike won’t be the exact size/style i like. They are nearly all city-bike style. Not many sizes are available, usually just small, medium, large (whatever that men’s).
It will be much heavier.
It will be more difficult to customise, for example with baby seat, lights, horns, mirrors.
It will probably be impossible to de-restrict, to be usable at normal cycling speeds.
Most importantly, it will not use standard parts. For example if the battery or motor dies after a few years, i have to buy a replacement from the same manufacturer, if that is even available. For a converted bike, i can change the components independently and choose any brand for the new parts (i think).
What is the argument for buying an off-the-shelf e-bike? Why would anyone do that?
You don’t have to build it, shops will service the motor (I’m sure there are some that will service conversions, but I don’t know any near me anyways), they may be more efficient (depending on the type of conversion/motor design you get), etc.
I’m with you, I’d do a conversion, but it’s something you’ll have to maintain and fiddle with where as the store bought ones will normally “just work” out of the box. No annoying rattles, no tweaking the motor position because it’s rubbing on something now, etc.