Well, there are gameplay reasons it has to be that way – if field moves were dependent on PP you could become trapped in certain areas and unable to return to a Pokémon Centre. In any case, for a lot of field moves it’s not like the combat and out-of-combat uses are actually the same or even similar – clearly we’re not supposed to imagine that using Surf (a special attack usually depicted as a powerful wave) involves vigorously swimming at the opponent with your trainer on your back. Even when the two actions are basically similar, I would imagine that doing something in the middle of a fight is a good deal more stressful and difficult than doing it any other time. Me, I think of PP as a vague measure of a Pokémon’s energy or exhaustion, the same way as HP is a vague measure of a Pokémon’s health or injuries. Obviously HP was never meant to be a realistic and precise account of the billion and one different kinds of godawful nonsense that can happen to a Pokémon on a daily basis; it’s just a number that answers the question “can I or can I not keep fighting?” and we accept that without thinking, even though it clearly doesn’t make much sense, because HP is an abstraction that literally everyone has been using for decades. PP is the same kind of thing.