Why validator slashings are so complicated in Eth2?

First, why don't slashed validators get kicked out of the validator set immediately, like in other proof-of-stake chains, and must exit in the next 36 days? Why they can't just exit now? Why can't validators exit immediately after getting slashed to avoid the special penalty? Why are slashing rewards so low?

On Optimistic Ethereum, 50% of the validator's stake is awarded to the slasher, and the rest is burned. The slashed validator gets nothing back on Optimistic Ethereum. Why don't you just burn whatever that is not awarded to slashers?

Why do you guys need years to make a proof-of-stake consensus system? If we used tendermint like polygon, we'd have proof of stake since 2018.