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Why does that matter to the government?It doesn't. It matters to the law, which is a different thing from the government.
And it matters because it matters to society whether someone with insufficient training and testing rides a huge motorcycle, or a truck, or whatever. Especially something like a sports motorcycle which needs more skill than an average car, and is much easier to slip (as accidents stats tell).
>Non-motorized cyclists can hit speeds of over 120 km/h
Rarely and in extreme conditions. Big motorcycles can do it trivially.
The world cycle record is 130 km/h, under very special circumstances and for just 200m or so. The highest end of the speed pros usually achieve is (according to wiki) around 70 km/h, quite less than 120 km/h. Not to mention a motorcycle (especially the kind more heavily regulated) has much more weight than a bike, and thus much more momentum even for the same speed.
>It requires skill to use a knife, and if it's mishandled it can cause severe bodily harm; should we require people to get a license to use a knife in their own kitchen?
I'm pretty sure people are allowed to ride any size of motorcycle in their own kitches or their backyard even. It's when they ride them on the road, and this can kill others, that it gets to be a concern for the law.
The same holds for knives btw. Carrying a knife in the streets in many jurisdictions is considered an offense, similar to carrying a gun. It sure is in mine.
And in general, for all over the US I read "(...) there are often limits on knives or bladed weapons that may be carried. Most states regulate an individual’s ability to carry a knife based on the following characteristics of the knife itself: The type of knife, including the purpose in which it is typically used for; (...) The length of the blade.".