You seem to lack a fundamental understanding of the two technologies, so I'll try to explain because I am inherently a patient person
Dedicated servers are typically run on commercial grade bandwidth, hosting providers usually have SLAs with their peers. A physical computer will usually run multiple dedicated servers but well within constraints, allowing for superior performance. This is an optimal solution.
A peer to peer host seed setup is typically run on a cable modem, DSL is much less popular in North America. This is consumer bandwidth with no SLAs. The computer "hosting" could be Johnny's Dell laptop or an Alienware machine. It could be filled with viruses, running torrents or be completely virgin. Either way, the person playing is also hosting and games are very demanding on system resources. It's not an optimal solution, it's a functional one.
In a setup where the client is hosting, the host has an inherent advantage over other players due to latency response. As in, the host has zero while everyone else doesn't. Another problem is the lack of reliability in consumer grade connections. Downtime aside, a dedicated server will usually provide a far better ping for most players connected, geographical/routing issues aside.
Finally, a peer to peer system is much easier to create cheats for. Data validation has to be done by a host or all clients, there is no in between. A dedicated server is again optimal here because it makes manipulation by clients much more difficult.
Your argument seems to be "Well it works and its not 99.9% verno hahahaahah". My argument is "Dedicated servers are far better for the vast majority of gameplay types". Even RTS games would be better suited by a dedicated server, it just happens that the gameplay is more accomodating to low bandwidth/latency scenarios which is why P2P is used more often.