1. because trains represent technology that is at least a century older than the automobile
2. because for a variety of reasons trains are not a necessity for most Americans
3. because in most cases trains are not economically efficient means of transportation and can only operate if the cost of operation - not just infrastructure but operation - is subsidized
so Americans rightly say - why bother
Trains are actually about a half-century older than automobiles, with the first example of the latter technology using the internal combustion engine emerging around 1886. And when it comes to experimental automobiles powered by steam, the first examples actually preceded the railroad locomotive by a few years.
The issue here from an American perspective isn’t really about necessity, but about market choice. The government has limited those choices for us.