The same reason Bitcoin doesn't implement every tom dick and Harry's supposed solution to every technical problem. The big boys have to get it right the first t

The same reason Bitcoin doesn't implement every tom dick and Harry's supposed solution to every technical problem. The big boys have to get it right the first time. Again, inertia. Ethereum does have proof of stake, a protocol called Casper FFG. It is currently live on the Ethereum beacon chain, waiting for consensus to move the entire network to it. There are lots of smaller projects trying different protocols they call proof of stake too, as well as different proof of whatever they came up with for whatever reason. Most of them are veiled centralization, some of them are interesting for one reason or another. I'm not here to shill networks or coins so I won't be tossing names around.