One reason is that the sea is the lowest point on the planet, so water naturally flows downhill towards it. Another reason is that the sea is a large body of water, so it can hold a lot of water. This means that rivers can carry water from far away and still have it all fit in the sea. Finally, the sea is always changing, so it can erode away land and create new channels for rivers to flow through.
There are also a few reasons why some rivers do not flow into the sea. For example, some rivers may evaporate before they reach the sea, while others may be blocked by mountains or other obstacles. Additionally, some rivers may flow into underground aquifers, rather than the sea.