In the modern oceans, there is a very high abundance of calcium and magnesium ions.

This keeps the phosphate concentration in the water low, because when there is much phosphate, it precipitates with the calcium ions, making apatite rocks.

However the oceans at the time when life appeared had a composition different than today.

The oceans have formed by the condensation of the volcanic gases, which are made of water and of acidic substances (mainly oxides of carbon and sulfur and hydracids of halogens and sulfur).

So the initial oceans were very acid. After formation they began to dissolve the more easily soluble rocks, e.g. carbonates and phosphates and then also the easier soluble of the silicates, starting with the alkaline silicates.

So the concentration of phosphate in the initial oceans was much higher than today, due to the very low pH of the water and due to the lack of calcium and magnesium ions in appreciable quantities.

In time the pH of the oceans increased tending towards the neutral value (today the oceans are slightly alkaline, but that changes with the increase of carbon dioxide in atmosphere, which makes them more and more acid), while the concentration of sodium and potassium increased, neutralizing a part of the acids.

Because the rocks with calcium and magnesium dissolve much slower than those with sodium and potassium, the concentration of calcium and magnesium in the sea water remained low for a longer time and it probably reached the current levels much later, after the apparition of life.

Only then the concentration of phosphate diminished to the current values.

So a much higher initial availability of phosphate in solution, in the sea water, is expected, it is not mysterious.