Because PoW is easier to implement correctly than PoS.

PoW can be as simple as checking a hash (in addition to the basic block rules). PoS has a lot more rules, for example Eth has a lot of PoS rules and includes features like slashing which improve economic securities. https://docs.ethhub.io/ethereum-roadmap/ethereum-2.0/proof-o...

When rules aren't by implemented correctly it causes situations like Bitcoin's 2013 fork https://bitcoin.org/en/alert/2013-03-11-chain-fork

Because PoS effectively means a handful of exchanges run the coin. This is far worse than a handful of mining pools, because pools depend on many more individual hash power providers.