Why is the former instead of the latter used?
вернуть are two related yet distinct verbs. "Вернуть" is to return in sense to give back something you've taken/borrowed beforehand. "Вернуться" is to be back. It looks like you've just checked some resource sort of translate.google.com, which is fine but not always provide you with all context needed.
May be it would be a bit easier to memorize the difference if you'll keep in mind following fact: "вернуться" is a reflexive verb — in Russian this verbs are ended with cя/сь. So it is roughly like "to return yourself".
By the way, this cя/cь ending is actually remnant of PIE similar (but more complex) feature. This is exactly what
se in reflexive verbs in Spanish stand for. Moreover, English word "self" is related as well.