Failed to excite?
Do you understand that it's more difficult to get a motorcycle license than one for an SUV? And the SUV comes with a 30x less chance of death (for the driver)?
That's why very few pick up cycling or motorcycling. It has a high barrier of entry and a stiff penalty if things go wrong.
-Crissa
It's almost exactly the same process to get a motorcycle license vs an SUV, the main difference is that minors can ride the motorcycle with a permit on their own, while in the SUV they need a licensed adult in the car with them at all times. When you have your permit, the license test for the SUV driver requires driving on-road with other drivers successfully, showing competency with both controlling the vehicle and the rules of the road. The motorcycle test is basically can you ride in circles in the parking lot.
Bikes are generally cheaper than cars and the insurance is vastly cheaper. The biggest barrier to entry is simply that most new drivers or riders are not familiar with a manual transmission, and to learn both manual and riding on 2 wheels at the same time can be intimidating.
I will grant that safety is a big part of the reason americans aren't big on motorcycles, but at the same time so many are happy to tap away on their phones while they drive, or get on a rental electric scooter drunk and crash at 20mph with no helmet, so our safety concerns aren't always based on actual logic and risk.
Back to the US though... the US is a strange sort of place where you are literally expected or even required to drive, everywhere. Nobody walks anywhere. It's weird af to anyone from Europe. So everywhere is likewise spread out in a huge sprawl, which means distances, so having highway capabilities is pretty important I should think, and 125s are basically utterly crap on fast roads with aggressive motorists everywhere. Or massive fun, depending on your outlook on life.
Cas 
I think this is true in lots of areas of the US, but it's not true for everyone and is certainly not true in denser areas.
In big cities like SF or NY it's easy to find people that don't own cars, who rely on other forms of transportation to get to work or the store. And it's becoming more common. Just read any one of thousands of articles about how millennials are "killing" the car industry. Owning a car is not what it used to be culturally, and more and more people are living without and seeking out other forms of transport. Add in that gig companies like uber have made it easier than ever to get a ride where you want when you want, and the desire to own a car, and to pay for insurance and maintenance, has plummeted for many.
Does that work in the boonies? No, of course not. But even though America is a vast rural land, the majority of our citizens are actually in dense urban areas, where getting around without cars is both possible and relatively popular.