Paying someone for a whole year gives you one chance of finding a full-time employee. Paying four different people for 3 months each gives you four chances.
Because the time on that particular job is not the only thing the company is paying for (with interns/co-ops). In fact, it's usually not the primary thing.
It's pretty rare that intern positions make sense in a strict dollar for time sense, you are better off thinking of them as an investment in the future. Accordingly, the pay rate is not directly and easily comparably to most jobs.