Are you asking how do lighting STAKERS make money if lightning transactions are so cheap? Right now they don't, people are doing it to bootstrap the network. But in the future, they will be able to make it on volume. Staking is nearly costless. There were 369 billion purchase transactions for goods and services worldwide in 2018 [1]. At a penny per, that's a lot of scratch.
[1] https://www.cardrates.com/advice/number-of-credit-card-trans...