This link[0] says that Europe had 77000km = 47845 miles of motorway in 2018. It's probably not quite as standardised as the US system, but it seems broadly comparable. See the second link[1] for a picture.
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].
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].
[0]: https://en.wikipedia.org/wiki/Numbered_highways_in_the_Unite...
[1]: https://en.wikipedia.org/wiki/United_States_Numbered_Highway...
[2]: https://www.fhwa.dot.gov/policyinformation/statistics/2017/h...