Because the only threat from a 51% attack is rewriting a couple blocks until they recognized as attackers and they get kicked off the network because bitcoin users actually validate blocks. In a PoS system there is an incentive for large stakeholders to increase block sizes. 1. It increases the usability of the network which increases the marketcap. 2. It pushes out smaller stakers who can’t afford to validate anymore because tx throughput becomes too high. If everyday people can’t validate the chain how can they fork it if they think the current block validators are acting against their interests? The ether account set will be far too large. So they’ll start from scratch.