There is nothing forbidding dating outside the same college in most cases, but it can be very impractical. At my school for example, Caltech, the work load was very high leaving limited time for social activities, only a small fraction of students had cars, public transit was not very good, and there weren't many places within walking distance of campus you'd be likely to meet potential dates.

At the time (~1980) among undergraduates Caltech was about 15% female, 85% male. There were probably some who did make off campus dating work but I never heard about it.

Even if a significant fraction could make off campus dating work, with that much of an on campus imbalance the ratio of the remaining males looking for an on campus date to on campus females would still likely be large and that's pretty annoying for everyone, but especially so for the women who if they were not currently dating were being constantly hit on.