According to the US Census Bureau's table of estimated population as of July 1 each year, there were 268.3M people 14 years of age or older, including 260.1M people 16 years of age or older, including 251.6M people 18 years of age or older, as of 2017.
According to Statista, there were 272.5M registered motor vehicles in the US as of the same year, the latest for which data is available. (Statista's source for that number is hidden behind a $588/yr paywall).
There are plenty of people who cannot drive, e.g. due to a disability such as blindness which prevents them from doing so. Census bureau numbers for 2017 list 7.6M with a vision difficulty, including .6M under 18 years old. The Census Bureau is the first source referred to on the National Federation for the Blind's statistics page.
Some cannot drive due to legal restrictions, such as license suspension, never having gotten a license in the first place, or long-term incarceration. There are also lots of people who live in urban centers or other places where they don't need to drive, and where it's not cost-effective to do so. There are plenty of others who would like to drive and for whom it may even be cost-effective, but who lack the capital/credit/capacity to afford a motor vehicle (especially once including insurance, maintenance, and fuel.) Some families also have a single (or small number of) vehicle(s) shared among a larger number of drivers.
Despite these factors, is there really more than one registered motor vehicle per eligible person, on average, in the US? If so, and if you'll excuse the pun, what main factors drive that statistical outcome?