There is a fundamental theory problem. That is, with PoW it takes real world investments in finite resources that also have scarcity built in. In PoW, a intangible construct like a blockchain, is secured by the physical manufacturing, shipping, consuming, of real devices. You have the cost of fiat based on supply/demand in purchasing the hardware, and the fiat cost of electricity for operation. These things are all real work, equipment, and costs in giving this made up blockchain a real, secured, immutable, construct
In PoS, you are using the thing that needs to be secured, to secure thing itself. You are securing an intangible string of 0's and 1', with the same string of 0's and 1's. There is the argument that in PoS the value comes from the fiat invested in the secondary market, and ultimately PoW comes down to how much fiat you have anyway, but this takes all of those other markets and scarcities out of the picture.
PoS all you need is more of the thing that needs to be secured. There is also the competition problem in that lets lay you have 10% of the staked value, you will be getting 10% of the rewards, and maintaining your position without adding any extra security to teh network.