Why do plant seeds not germinate well in waterlogged areas?
Most of the time, the fastest way to germinate most plant seeds is to soak them overnight or for 24 hours to speed up their sprouting process.
Although it is most likely true that seeds need moisture to germinate, even when they are submerged in the soil, too much of one thing is never good, especially for plant seeds.

Why Seeds Do Not Germinate Well In Waterlogged Gardens
Why Seeds Do Not Germinate Well In Waterlogged Gardens
If there is no moisture, the seed does not germinate, may survive and germinate at a later point when water is abundantly available if other conditions allow.
On the other hand, if there is too much moisture, the plant seed may germinate or start to germinate, but when the plant seed is overly moist and saturated with water, it is unable to grow further and shrivels up and dies.
A waterlogged seedbed has no potential for future growth because the germ of the seed is dead because of too much water.
Proper Moisture
The proper moisture level is mandatory when it comes down to plant seed germination. The seed that germinates has adequate moisture for growth and puts down root structures as well as top growth.
Eventually, they will have sufficient stores to possibly survive periods of inadequate moisture or nutrients.
Too Much Moisture
If there is too much moisture in your starting cells, the seed will become oversaturated, and the germ of the seed may start to grow; however, it is likely it will rot before it can grow roots and top growth.
Nevertheless, with too much moisture, plant seeds will probably germinate, but they will never grow to the point where they will become healthy and strong to transfer.
So, at the end of the day, it is always ideal to have moderate moisture when you are showing any plant seed. For faster germination, it is always ideal to soak them overnight before sowing them in the raised bed or directly in your garden.