There's more to it: it's irrational to return a wallet to somebody you'll never meet/see/extract value from... but not doing that can injure your own self-image
Often doing "the right thing", in irrational circumstances, is nevertheless self-protective.
Otherwise you get friction in your social programming that you might never overcome.
You can also brag to people about how great you are because you returned a wallet and expected and/or received nothing in return. With the expectation of receiving social approval.
FWIW, you don't actually have to return the wallet to claim you returned the wallet. In fact, you don't even have to find one. The fact that lost wallets get returned makes me think telling people about it is not the motivation.