That's incorrect. There are plenty of chains with pure PoS, and the reason it's taking Ethereum so long is because they have an established PoW chain that they are trying to seamlessly transition to PoS. At the same time they are also working on taking into consideration more aspects like how it works with their platform/smart contracts and sharding. I'm sure there's many other considerations I'm not aware of as well. All other pure PoS implementations are much simpler as they're just simple payment blockchains which greatly reduces the difficulties and moving parts.