Why would I run a Malairte node if it does not earn coins?

A node does not earn MLRT, but it makes the network stronger. By validating every block yourself you stop trusting third parties, you help relay valid data to other peers, and you make the chain harder to censor or attack. More independent nodes means a more decentralised, resilient network. Many people run a node on a machine that is on anyway, as a quiet contribution to the project they mine or hold.