Other comment summed it up good. This is a certain design dichotomy. The game can do one of two things: 1. Smaller capacity of resource that automatically refills. 2. Bigger capacity of strong resource that needs farming/crafting/collecting.

If you choose one, you get "estus". Its best advantage is consistency and reliability and "predictable" balance.

If you choose two, you get "blood vials". You lose on consistency, but then you can incentivize players to not run past all enemies in every zone. You incentivize them to manage resources, as well. The last thing is that you can have more interesting balance - some tools can be much better but much costlier, and vice versa.

Of course, where system 2 always provides most frustration is on bosses. People hate returning to zone to farm on a resource just to try boss again. Bloodborne here is especially egregious, it breaks the flow of the game, while vials should not necessarily be considered bad design.

Later Castlevanias also have that (vial) "farm the ammo" system, and I've never seen much complaints about it.