I think it's more that measuring engagement is cheap: just add some metrics to your app. Measuring usability is expensive: you need to give large groups of use

I think it's more that measuring engagement is cheap: just add some metrics to your app. Measuring usability is expensive: you need to give large groups of users your thing and be physically present to see how they try to interact with it. I would be fairly surprised if startups regularly tested their UX the way e.g. Microsoft (used?) to test some of their products: give it to a room full of old people who have never touched your app and ask them to do something with it.