You're comparing all European motorways to the largest category of motorways in the US. Not all motorways in the US are classified as part of the Interstate Highway System, even though they're connected to them. The US also has many motorways that are part of State Highway systems[0], and the Federal Numbered Highway system[1].
If we're talking about all motorway style roads, there's some additional roads in the US that qualify: 67,353 miles or 108,394km[2].