An enormous percentage, especially in the current housing market, however…
Many (most?) American cities have wildly inadequate public transit and are prone to sprawl. Many Americans live in apartments, but are a multiple mile walk from their grocery store. If there’s any public transit at all it’s probably an infrequent and unreliable bus line that may not go anywhere near their home to begin with. They live in apartments, but are not anywhere near ‘downtown’.
These are problems that need to be solved, and quickly, but public transit is best grown with a city, which didn’t happen. Inserting a subway after the fact is difficult, expensive, and slow.
The reality of right-now (which is all a renter is likely to be able to consider financially) is that a reliable car is an essential item in most parts of the country.