Peering is a bit of a problem - but lots of services do peer, albeit using prefix-codes for dialing, so the caller needs to know they want to peer, and care.

Extra revenue: large services (with their own PSTN gateways, not ones who re-use Level3/etc) pay money to complete calls over PSTN, and get money for completing calls. If they don't have PSTN gateways at the destination exchange, they pay some LD charges when completing outbound calls (depending on the deals they have with PSTN/LD carriers). So, overall net, calls over PSTN are a net money outflow (unless you're charging for LD by the minute), so there's a reason to peer.

Depending on deals, smaller companies that outsource their PSTN gateways (i.e. most VoIP companies) gain nothing or very little for "unlimited" customers, and perhaps gain little for non-unlimited customers. If the company pays by the minute (i.e. the "unlimited" part isn't borne by Level3/etc), then the VoIP provider has a real incentive to peer.

Another (mostly future) incentive to peer is wideband audio codecs and video calls (i.e. Ojo).