It isn't just about carrying it up the stairs to your apartment, it is the stairs taking the subway and getting off, getting on and off buses, going up and down stairs for parking garages when you get where you are going, the battery lasting a little longer overall, taking hills and accelerating a little faster. Especially for someone who is not a typical fit cyclist, the weight difference can be a the difference between this and not cycling at all.
For relatively short trips, mixing with public transit an electric kick scooter is a competitive option, cheaper, comparable weight or lighter. Slower and less utility though
These don't really seem niche. The lighter the better, simply because F = ma, the lower the mass, the higher the acceleration from the same amount of force from the battery applied. Or, higher range.
The 1-2 kg weight savings is useful if you need to carry your bike up the stairs to your top floor apartment. Or to ride it up hills in manual mode. Both of those seem like niche cases.
Edit: other responses tell me that modern carbon fibre frames are not much more expensive than metal, not much more fragile, and may be more comfortable. That makes it a more reasonable choice than I thought.