"why do social networking platforms [..] adopt much stricter password policies?"
You have insurance for your money.
There is no insurance for your reputation.
People will not care so much for the safety of their bank account since they can have a discussion with their banker face-to-face, prove their ownership and innocence, and get things set straight.
Try doing that with a Facebook representative when your account has been hijacked (don't know if that has ever happened).