Moq, very well said again.
The shidduch date is screened, regulated, and vetted generally. This removes much opportunity for sin. The daters knows if they try anything stupid, word may get back to others (ie the Shadchan who is involved.) Furthermore, both parties in a shidduch date are serious about marrieage generally; far less clear when bumping into an opposite gender at a party or college.
Additionally, when meeting up with an opposite gender at a party or college, there is a propensity they will “just be friends”, rather than be considering marriage. It is an issur m’doraisa to “just be friends” with a person of the opposite gender. (See Igoros Moshe and others.) No one can seriously claim that even in a “frum” wedding or college where boys meets girls, many don’t become “just friends”, where it would never have happened if not for the mixed setting.