Why is a validator punished for downtime in Proof of Stake?

So in ETH PoS scheme (and in the PoS scheme of other cryptocurrencies), staked tokens can be slashed if the validator is offline for too long. But why? Why can't the network be designed to be resilient to such failure? In Proof-of-Work, no miners or full nodes get punished if they just stop.

I'm concerned about this because this "always-online" requirement scares away most people from running a validator on their own (that plus the 32 ETH is too expensive). This encourages centralization.