Why does overwatering kill plants?
The most important reason for plant death due to overwatering is that it restricts or even cuts off the oxygen that the root system depends on for proper functioning**.
We all know that plant leaves extract carbon dioxide from the air to make their food, but they also need oxygen to maintain healthy growth. At the roots of plants there are fine hair-like roots, which absorb water and nutrients from the soil for the plants. Root hair cells are all modified plant cells and, like all plant (and animal) cells, they need oxygen to metabolize. Whereas, in the soil, oxygen is stored in tiny air pockets around the roots of the plant. If you water the plant too much, it will destroy these small air pockets and make the oxygen disappear, which means that the root hairs cannot get enough oxygen from the soil to metabolize and eventually "suffocate" to death. When enough root hairs die, there is a situation where a lot of water is watered, but the plant still dies. In addition, too much water can also cause the plant roots to become eroded by fungi, leading to root rot. Whether it's a lack of oxygen or root rot, the root system can't provide the nutrients and water necessary for the plant to survive, killing the plant.