Ask HN: Why do so many iOS apps require subcriptions?

I recently switched to iPhone and while most things have been great, the app ecosystem is a bit heavy-handed with monthly subscriptions.

I tried searching for a sleep cycle alarm with a noise machine, but almost every app only functions as a basic alarm unless you subscribe to their Pro package for $5.99/mo, or in some cases, will not let you access the app at all! I've seen this happen with things like calculators as well and while I'm fine paying for an app, this seems to be a strange use of the subscription model.

Is iOS app development so expensive to develop/maintain that it requires this sort of revenue model?

Are there other reasons why this has become the norm in a lot of iOS apps?