Because the design goals for each are different. Run your own chain vs a cohesive integrated system
Polkadot, my impression of it is that you deploy your own parachain and can customize it and it anchors into the main chain vs eth2 where all the chains/shards are the same Seems to me eth2 is trying for better composability across the system, but both may end up similar
The general design paradigm of Cosmos is different from PolkaDot, it doesn’t have a relay/beacon chain and doesn’t require shared security. Also, Cosmos already has fully functioning code built on the Cosmos SDK framework.