What's wrong with following motorcycle regulations?

Because owning a motorcycle is a huge pain in the ass on account of motorcycles costing a decent amount of money, weighing 300lbs, going on the highway. If a $1000 ebike can only hit 40mph and weighs less than 100lbs, why not let people just buy them and ride them with a normal drivers' license?

Because e-bikes have effectively done regulatory arbitrage and the sky didn't fall. You want more people using small electric vehicles where before they would have used a car, you lower the burden to get one on the road.