why do plants die in waterlogged soil

Basically, when a land-based plant is water-logged, the roots begin to rot, thus killing the plant. The root system of a plant is just like a "brain" and without it functioning the plant simply can't survive. Some plants need to be in a "well-drained" position. This is because there are some plants which rot quickly and they need to be drier around the root system.
Fully submerged aquatic plants use their roots mainly for anchorage and as runners for proliferation.

Since the leaves are submerged, they do not have the ability to evpotranspirate, therefore, the plants rely on foliar feeding.

Floating plants, such as Water Lilly, Water Hyacinth and Duckweed do feed through their roots and have stomata on both sides of the leaf, unlike terrestrial plants where the stomata are found only on the underside.

This allows for breathing and evaporation. I dont know much about hydroponics, but from what I gather, in hydroponics, the water is kept aerated by the continuous movement of the water or by an air pump and diffuser (as in an aquarium) this may ensure that the roots are partially protected from the water by air bubbles forming on the root system.

In static systems I think the plant has to be removed from the water occasionally for oxygen to get to the roots. If a terrestrial plant was to be kept in waterlogged soil, the weight and compaction of the soil coupled with the stagnant water would deprive the root system of oxygen.

I wonder if a good supply of air in the water, which might protect the roots from being too wet, might also prevent the plant cells from becoming waterlogged.