Because their soil and infrastructure can’t handle rain when it comes. Either directly or from lands and mountqins that lie ‘upstream’.

The capacity of air to hold water increases greatly with temperature. Hot air over large bodies of water can gather up a huge amount of water vapor, whereas the cold air over, say, the North Sea picks up relatively little.

When that saturated air moves inland and encounters colder air coming from the continental landmass, or is forced upward to cooler elevations by mountains, all of that water vapor is forced out of the air in the form of rain. The same thing happens in cooler climates, too, but there just isn’t as much water in the air to condense.

Monsoons and tsunami’s, mostly because of the tutonic plates shifting under the ocean causing earthquakes as well as volcanic activity also in the vented flutes at the bottom of the sea floor. It can be that landscapes are altered due to the constancy of these events over time that tend to reshape the overall landscape to create wetlands that are more prone to flooding.

Could be low elevation as well. If something is close to sea level, it will tend to have a different infrastructure in lieu of the type of soil and rock that makes up the crust of the landscape.

Any place with too much water will flood, including low places like Bangladesh and India around the Bay of Bengal, rivers such as the Red River in North Dakota and the Mississippi along its entire length – and the Indus River that flows through Pakistan or the Mekong in SW Asia – and even “high country” can flood when hit with too much rain, too fast. Flash floods in the Rocky Mountains are common during the rainy season, or at any time a thunderstorm dumps a lot of water on high, rocky ground.

It’s just that along major rivers during rainy seasons (and especially the “lower” reaches of rivers as they reach their estuaries at lakes and ocean shores) that it’s a seasonal thing, and it displaces a lot of people because… that’s where the people live. And when a lot of people are displaced due to flooding, that’s when it makes the news and that’s when you hear about it.

But it happens pretty much annually, whether you hear about it or not. Some years are worse than others, too.

Not so hot countries flood too. Flooding is the biggest killer of natural distasters. Too much rain where there are rivers and lakes cause flooding. Hot climates tend to be closer to sea level, so the rains cannot run off as easily. Although in the tropics, when there are mountains, mudslides can result, which are devastating.

Also, there tend to be a lot of people living close to the water – and a lot of people in general – where the climate is warm.

Nearly all Tropical Climate countries flood, always had.