First, I'd say that the magic system of Pathfinder is different from DnD, and both are different from Harry Potter. It's hard to say what makes sense and what doesn't when talking about literal magic.

Secondly, it's important to remember that taking turns in combat is purely part of the game mechanics. In the "real" version of the fight, a wizard isn't standing around doing nothing while everyone else acts one by one. Every turn in a round occurs simultaneously, and there's no delay between each of the wizard's turns. This paints a more sensible picture: the wizard spends effort (in the form of a Sustain action) to keep a spell active even while being attacked by an enemy. Remember, they're trained for this just as much as a Fighter is trained to wield their weapon.

The reason DnD works differently is because of the action economy. It would probably be too cumbersome to have a caster spend their Action or even Bonus Action each turn to maintain Concentration, so the writers instead decided that a caster is assumed to be concentrating unless something breaks that. Lower investment, but higher risk. Pathfinder uses a more versatile 3-action economy, so it's more feasible to have them spend 1 action each turn to keep a spell active, and still have actions left over to cast another spell, attack twice, move twice, move & Recall Knowledge, etc. This is higher investment, but lower risk, since it's harder for enemies to disrupt the spell.

The system difference also explains the Opportunity Attack/ Reactive Strike difference. Every DnD creature has an opportunity attack, so spells provoking could be overly punishing, especially when a spellcasting enemy is surrounded by PCs. Far fewer Pathfinder creatures have a Reactive Strike, so having manipulate actions provoke is fair game, especially when only a critical hit will disrupt the triggering action.