Most modern flat roofs (at least in my neck of the woods) aren't actually flat. They're at an incredibly shallow pitch. In residential applications (after underlayment) we use high density foam slats that get put together in a grid formation, each piece gets slightly narrower on one end until they can properly fit under a piece of bird stop around the fascia. Before the bird stop goes on, the foam is sealed with a waterproof self adhering material that comes in huge rolls. When properly installed, they can last 15-30 years. Just as long, if not longer, than any shingle roof. There's also high density polyurethane foam flat roofs that can be sealed and are able to last up to 50 years. Many commercial operations use these methods as well as hot tar mopping, which can basically last forever. The one drawback to flat roofs is load bearing difficulties in places that snow. I don't know much about that but seeing as flat roofs are everywhere in the north too, it must not be too difficult to work around.

Don't get me wrong, I love a good tile roof too, but there's plenty of beautiful structures with flat roofs out there. And tile has considerable draw backs as well. Tile is incredibly heavy, they're very fragile, they're ludicrously expensive, and they're expensive to maintain.

On the rain, no roofer is working in the rain, and tile roofs are vulnerable to rain during construction too. When it's down to the plywood, you're as likely to have rain troubles as any other roof. You can lay shingles and foam for flat roofs when you're dried in too, but no one will if it's more than a drizzle. Tile becomes an active danger in the rain because most tiles get really slippery in the rain

Replacing broken tiles is a bitch and a half, you have to carefully dance between tile joints and one slip of your foot (which is likely during repairs because tiles are slick and can become moldy/slimy when not maintained properly) you're likely to break another tile, leading to more work and more chances of another broken tile. Even the process of removing a broken tile can break the tiles surrounding it

Tile in general is really finicky, high maintenance, and requires unique tools, methods, and skilled laborers that are capable of doing the work. That is by no means a bad thing but it's really easy to get a flat roof that does the job perfectly fine. Tile, not so much