y not,
Apparently, finders keepers is a form of Theft by Finding. It’s even punishable by law in some countries.
Sometimes it’s not so clear whether it’s right to do it or not. If you picked up cash on the street, it’s yours. There’s no way to determine who dropped it. But if you picked up a wallet containing cash and identification card (not necessarily ID card but any form of traceable identification e.g bank card, employment pass etc.), then you should make an effort to return it to the owner. i.e. make a police report.