Subscription models only make sense for an app/service that have recurring costs. In the case of Lemmy apps, the instances are the ones with recurring hosting costs, not the apps.
If an app doesn’t have recurring hosting costs, it only makes sense to have one up front payment and then maybe in app purchases to pay for new features going forward
Everyone wants a constant revenue stream, app devs aint unique. And also like everyone else, they charge what people are willing to pay. Price is never about cost.