Why was earth's water not broken down by sun rays?

Look up “the water cycle”.

It’s called the Hydrologic Cycle.

Water in the air is called vapor, when it comes together in a form we can see that is called condensation. This forms clouds. The water then comes down as precipitation ( rain, snow, sleet, hail). This water then falls into bodies of water (oceans, lakes, rivers, streams) or the water falls to the ground. That water either runs into bodies of water or is absorbed into the ground. The water in the ground is either taken into the roots of plants or seeps down further into the ground. There is a layer of porous rock called the aquifer. The water gathers there forming something like underground lakes. This is called groundwater. This water is either brought up by us through wells or comes out on its’ own through springs. The water that is absorbed by plants travel through the plant and is expelled through their leaves this is called transpiration.

And when the water in the bodies of water like River, lakes, streams, rivers, ponds is heated by the sun is becomes water vapor AGAIN. Starting the whole process over AGAIN.

There is no new water on Earth. The water we have today is the same water that has been here from the beginning of time. That water in your store bought bottle was dinosaur pee at one time. ;)

The amount of H2O that is transmuted or otherwise undone by solar radiation is statistically insignificant compared to the stable H2O on Terra.

Well…. there’s alot of it.

At the physical level, the water that evaporates eventually recondenses as precipitation. At the nuclear level, solar and cosmic radiation convert tiny amounts of the hydrogen in water vapor to deuterium or tritium (one or two neutrons in the nucleus vs no neutrons in normal hydrogen) but this does not affect the hydrogens chemical properties. Tritium is radioactive, but only a weak beta emitter with a 12 year half-life; so little of it exists in nature that it poses no health hazard.

Initially not enough solar power to completely “evaporate” all. The evaporation of H2O Created the atmosphere which now protects it..
The drying of the earth is inevitable but you shouldn’t worry about it :-)