signing a block. requires no work/cost
thus its simply a game of who has decision power (who do people follow)
EG
if people decide to follow a block the main thing is the why.
in PoW its the not just
a block that reaches most nodes the quickest
a block that follows the rules that is the quickest
a block that everyone then agrees they seen first thats valid
but also in PoW that block has to have done X work, which:
is not just about cost. but also an extra layer of puzzle complication, which not any random/chosen person can do in milliseconds(unlike PoS)
PoS does not have this extra layer which means chain re-orgs are both cheap and easy to perform.
heck PoS is just a 2 layer of "fastest first" they are always in a constant state of re-org. they have several possible blocks that are quickest seen. and then confirm when one of them gets far enough ahead of a threshold height.
on a PoS chain it becomes easy to perform them by, for instance easily changing code to have the initial peer connections to connect to specific DNS which then tell peers to follow certain nodes. which then control which relayed data to see first
where as in PoW if a certain DNS listed certain peers. those peers data that connect to selected pool(s). still need to perform expensive work to even have a block thats acceptable to the network. thus cant be easily manipulated to just accept some "quickest first" peer of their choosing