I'd venture to say that stunningly few people actually actively thought "I think I'll go and ask someone to mentor me" and then went and did it because of all the reasons you say and more.

I would imagine most mentorship is of the incidental kind that just happens as you work with colleagues and some of them teach you things that take you to places you wouldn't necessarily have gotten to on your own. You may in turn do the same for others without realising it.

Definitely working alongside good developers naturally benefits you greatly.