In the United States, vehicle ownership is done at the state level, not the national level.
Each US state has its own process and requirements, and many of these requirements are not friendly to international visitors.
20+ states require a US drivers license or US social security number, 12+ require proof of residency (bank statement, utility bill, etc.)
40+ states have vehicle inspection requirements that would be very difficult to meet if you wished to renew your bike’s registration while it’s outside of the US.
And then there are sales taxes - ranging as high as 10% in some places - which are due in the state in which you register your vehicle.