I would believe that the reason expressways aren't tagged and rendered the same way as motorways would be that it's very contentious among the community what an "expressway" is, granted that they can vary between a divided highway like US 95 north of Las Vegas/south of Boulder City, a New Jersey-style expressway that's divided and has interchanges but retains driveway access, a super-two like Route 7 around St. George, UT, or an undivided high-speed road limited access points like Route 9, also around St. George. This is opposed to a motorway/freeway which is more straightforward in concept; controlled-access, divided, and high-speed. This ultimately results in examples like the Route 73 Wilmington, Ohio bypass, which on paper is as important as the primary routes at each end of it, but it is characteristically a fully-controlled-access freeway.

The good thing about the expressway tag is that, while it doesn't really render on many map tiles aside from Americana as you mentioned, it does affect routing—which is just as if not more important than how it renders.