The number one cause of teen fatalities in the United States, for a while, has been motor-vehicle accidents.

In an effort in increase safety for teens (and other road users), every state in the U.S. has adopted a form of Graduated Driver Licensing. These laws have been shown to significantly reduce motor-vehicle fatalities among teens, and as a consequence they also increase safety for everybody else on the road.

Motorcycles in the U.S. have been shown to be about 26x more lethal than cars for the operator, per mile driven. We don't have much for motorcycle specific graduated licensing here, but other countries do, and I'll leave it up to you to do the research to see whether or not their laws are effective.

Almost every activity that involves a substantial amount of physical risk requires you to acquire skill in increments. I can't just buy a plane or helicopter and fly it around, or solo skydive, or put an 8-year-old behind the wheel on public roads.

The CC of the motor and HP/weight ratio of the bike are objective, enforceable ways for regulatory bodies to control what kind of mechanical power people are responsible for. Why? Because more powerful, quicker machines can make things happen faster, and if your skill level can't decipher what your body is sensing, you make mistakes, and people get fucked up.

I think you have a point that there a lots of people who could be fine on bigger bikes on their first day, but there are a lot of people who would hurt themselves or others. In other countries the system means the people who would be fine on the big bike can still ride, and the people who couldn't handle the big bike get to learn on a more manageable machine to increase their skill. Since everybody shares the road I think it's generally considered a win-win, and it's also good general advice to give to other people considering you have no idea where they fall on the "Will they kill themselves" spectrum.