I agree there's a payer/benefit mismatch, since filtering your own egress only protects other people's networks. But I would've thought it would still end up being incentivized somehow through things like peering or transit contracts. I don't have a good insight into how such contracts work, so I could be way off. My assumption was that in at least some such situations the other party would care if you're feeding them bogus stuff through the link, even if you don't care, so they would be motivated to require that you do egress filtering on your end as a condition of getting the link.