A vault app that holds your photos on its servers has a bill every month. One that holds nothing does not. That single fact explains most of the pricing in this category.

Cloud storage costs the developer every month, for as long as your photos sit there. A recurring price is not a trick.
An app that stores nothing has no monthly cost to pass on. A subscription there is margin, not infrastructure.
A subscription only gets paid if you stay. A one-time purchase has already been paid. The risk is an app nobody maintains.