- oribanakagawa4 hete
Because the verb ”to go out” is the object/descriptor of the preposition, ”before”.
In this case we use the gerund form of the verb, the ”ing” form.
Now, if you have a sentence like this ”Before you go out, have something to eat”, then ”Before” describes ”go out”.
The difference in meaning between ”Before going out” and ”before you go out”, the emphasis is on the time ”Before” in the first case and ”You go out” in the second case.