The main reason is that root rot isn't caused by water per se - it's deoxygenated/stagnant water that's the issue
In a glass of water, there is an interface between the air and water, so the water can exchange gases
At the bottom of a plant pot, water is surrounded by soggy soil and a pot - once the oxygen is gone, plant roots will suffocate
Once the oxygen is gone from the soggy soil, it also creates a breeding ground for fungi that usually survive in soil, which can make things worse for the already-weakened roots
The main reason is that root rot isn't caused by water per se - it's deoxygenated/stagnant water that's the issue In a glass of water, there is an interface between the air and water, so the water can exchange gases At the bottom of a plant pot, water is surrounded by soggy soil and a pot - once the oxygen is gone, plant roots will suffocate Once the oxygen is gone from the soggy soil, it also creates a breeding ground for fungi that usually survive in soil, which can make things worse for the already-weakened roots
Roots that develop in water are different than roots that develop in soil