Why flying boats solved an infrastructure problem

Why were flying boats so common before the Second World War?

They could operate over ocean from harbors and improvised coastal bases without the time and land required for a large runway.

Time, geography, and existing infrastructure mattered more than the price of concrete alone. A flying boat could use a harbor or sheltered coastline, and a seaplane tender could provide a mobile naval air station where permanent facilities were sparse. Water landing also offered an emergency option during long over-ocean flights. These advantages kept flying boats attractive wherever dispersed maritime patrol mattered more than runway performance.