It is very easy to buy a car, motorcycle, boat or any vehicle in the USA, but it is very difficult to legally drive the vehicle in the USA after you buy it.
In order to drive any vehicle on the public roads, you must have a valid license plate for that vehicle. In most states, you must first register your ownership of the vehicle with the state motor vehicle registration bureau. To do that, you must have a permanent address in the state, and you must have valid liability insurance. Only then, will the state issue to you a license plate for the vehicle.
In some states, you can obtain temporary license plates for a vehicle if you intend to export the vehicle out of the state. These temporary license plates are usually valid for no more than 30 days. It is assumed that you will take your vehicle and have it registered in another state or another country before the 30 days have elapsed. This works well if you only drive the vehicle for a few weeks, then ship it to another country. If you want to tour the USA for an extended period, then the temporary license will expire before you have completed your tour.