The primary reason why most panels don't do this is longevity. A panel is designed to be put onto the roof and stay there for 25 years, it is entirely solid state and its weather hardened to survive hail, wind and lots of sunshine. The moment you introduce water to just the panel you make the panel more efficient but you also drastically increase its failure rate, water corrodes and over 25 years its much more likely to cause a failure. On top of that the pump is a moving part and its going to require replacements over the lifetime of the panel.

In many countries around the world Solar heaters for water are quite common. Using tubes to heat water you can get near 99% efficient capture of the suns energy to heat which compares very favourably to the 20% of a solar panel to electricity. But their rated lifetime is quite a bit shorter and their pay off period looks quite different as a result.

So while there are benefits to combining them it increases the cost considerably and decreases the longevity and it just doesn't make much economic sense most of the time. Besides on a hot sunny day your panels are going to do great anyway, its clouds that really reduce the collection.

As an example my system will give me 34KWh on a cool 100% sunny day but on a hot day its more like 31KWh. But on a cloudy rainy day it can get down to 7KWh. The heat isn't as big as an issue as its made out to be.