Not a climatologist, but I would presume a lot of it is because there also happens to be a lot of water there. For one, the rotation of the earth kinda pushes the oceans into a squashed ball shape, so more water ends up at the equator. The distance to the core at the equator is roughly 13 miles greater than at the poles.. But also, in our current state of tectonic drift, there’s just lots of ocean at the equator surrounding various bodies of land. And the extra heat due to more direct sunlight for more of the year means more evaporates from oceans and enters the water cycle. All it needs then is to meet colder air above land, and we have lots of atmospheric currents that end up helping with that, and those areas tend to be significantly wetter.
For another, possibly better-researched answer, here’s an answer to a similar question on Quora.
Not a climatologist, but I would presume a lot of it is because there also happens to be a lot of water there. For one, the rotation of the earth kinda pushes the oceans into a squashed ball shape, so more water ends up at the equator. The distance to the core at the equator is roughly 13 miles greater than at the poles.. But also, in our current state of tectonic drift, there’s just lots of ocean at the equator surrounding various bodies of land. And the extra heat due to more direct sunlight for more of the year means more evaporates from oceans and enters the water cycle. All it needs then is to meet colder air above land, and we have lots of atmospheric currents that end up helping with that, and those areas tend to be significantly wetter.
For another, possibly better-researched answer, here’s an answer to a similar question on Quora.