What does it mean to say Layer 2s don't need to implement a consensus, and inherit it from Layer 1?

For example Ethereum has proof of stake on its L1, why is it that a L2, being a whole new chain, doesn't need to implement its own consensus?

Is this because of the centralized sequencer, i.e. only one block producer in L2s thus no need to have block proposal mechanisms?

If that's the case, won't L2s have decentralized sequencers in the future, having multiple producers and in fact needing a consensus layer of their own?