Plenty of people do. Personally, I have ridden on I-90 in MT three times (every inch of Interstate Highway in MT is open to bikes), I-94 in ND, I-84 in OR (virtually all of I-84 in OR is open to bikes) and I-80 in WY, not to mention numerous U.S. Highways, some of which were divided with limited access. I also rode the equivalent of an Interstate Highway heading towards Sevilla, Spain. In the case of I-84, I-94, one stretch of I-90 and I-80, they were the only paved options available.

It's not always as crazy as you might imagine. The stretch of I-94 in ND was empty at 7 a.m. The stretch of I-80 in WY, which is part of Adventure Cycling's Trans Am route, was also not very busy the morning I rode it. The biggest PITA about Interstate riding can be the shoulder debris. Got to watch for things like shredded tire treads and hub caps. However, the shoulder is often wide enough that you can easily steer around them while remaining on the shoulder.