Proof of Stake and 51% Attacks

I've been reading up on Proof-of-Stake for ETH2, and had a question on 51% attacks. The docs I've read suggest that such an attack would require owning 51% of all ETH, and so attacking the integrity of the system would be counterproductive since they're already heavily invested in the coin.

What I don't understand is why such an attack would require 51% of all ETH. Wouldn't it just require owning 51% of all validators? The amount of ETH tied up in validators will surely be only a small fraction of the total ETH in circulation, will it not? For example, if there are 250k validators staked at 32 coins each, that's 8M ETH. But that's less than 7% of the 120M ETH in circulation. So wouldn't an attacker just need to spend 8M ETH to create 251k validators, and control the entire 120M network?

What am I missing here?