That works because Chicago is in the north central part of the country, where highway numbers are firmly in the double digits. In the southwest the numbers are often single digit, so it's awkward and confusing to use without a preprended "I-" or "the".
To ultra-pedant it up, generally only odd prefix digits indicate a spur (only connects to the prefixed highway in one spot), where even numbers indicate a bypass or beltway (which intersect the main highway multiple times).
