Well, because it has its own chain with its own consensus, it does not depend on the security of the main chain. If it merges with the main chain it is no longer a sidechain. Polygon is another example of a sidechain.
Sidechains that don't periodically merge, are no sidechains but just forks of the main chain, no?That would be terrible. Especially without PoW, there would be no decentralized on such 'sidechain' (fork-chain
) either, right?