Fog Creek has frequently had co-ops; they just don't advertise it very much, for two reasons: they can't offer the same outings/bonding experience, and they can't take as many students at the same time. That said, if you're interested, apply. They've had some amazing co-ops over the years, and I promise they'll take your application seriously.
Fog Creek actually does occasionally do co-op classes during the fall/spring, but there is rarely (never?) more than one at a time and we don't really advertise it.
At my university, we get a lot of co-ops because of the engineering nature of majority of the students. I've found that engineering companies that make physical things are more likely to do co-ops and companies that focus on a completely coded product/companies that have a traditionally CS focus tend to do interns. Just my experience here though.
Second this - I think companies that offer year-round internships set themselves way ahead of the pack and get to choose from a very qualified and talented pool.
I didn't attend the Rochester Institute of Technology, but friends of mine who go there tell me that co-ops are a mandatory part of the CS program there.
Waterloo's "co-op" system is quite different. The whole undergraduate co-op program lasts about 5 years, and you alternate between 4 months in school and 4 months working throughout (i.e., you don't get summers "off"). This allows the students to try many different companies of varying size and culture.
I'd assume having year round interns and a continuous recruitment process would be less disruptive to the team's work velocity and give you a bit bigger reach for students too.
Plus, I'm a bit jealous of some of the summer-only internships at a lot of interesting companies. Can't complain about graduating with 18+ months of interesting work experience pretty much guaranteed though.