Why are off-chain transactions cheaper?

Could you elaborate on why off-chain transactions tend to be cheaper compared to their on-chain counterparts? I'm curious to understand the economic incentives and technical underpinnings that drive this cost difference. Off-chain transactions, as I understand, occur outside of the main blockchain ledger, allowing for greater scalability and reduced congestion. However, I'm interested in hearing the specifics of how this translates into lower transaction costs. Could you explain in simple terms the key factors that contribute to the cost efficiency of off-chain transactions?