WP Engine went all-in on WordPress. They promote WordPress as a secure, scalable and comprehensive solution, which builds global trust in WordPress even if you don't use WP Engine for hosting. They help people set up new sites on WordPress and migrate existing sites to WordPress. They get people using plugins, where people pay for plugins, give feedback, and give bug reports. By allowing hosting with custom plugins, they bring extra customers to plugin creators. By increasing demand for WordPress, they bring in work for WordPress site developers. All this feeds into the ecosystem that helps everyone.
Just because they're not literally giving money to WordPress doesn't mean they're not helping the ecosystem and to say otherwise is really shortsighted. I'm sure there's plenty of people reading that have made tens of thousands of $ or more from providing WordPress services that don't give money directly to WordPress too.