I see your point. Again, that's a problem that has a lot of different variables. First of all, any solution must be particular to the local conditions. Los Angeles is different from Seattle is different from Charleston is different from Paris is different from Venice (and so on). Second, different species of tree have dramatically different requirements and live in wildly different conditions.

For example, The American Southeast is full of live oaks, which provide lots of shade and are well suited to the environment there. Those trees wouldn't do as well in, for example, desert climates in Arizona; however, the Palo Verde tree thrives in that region, and is used as a street tree that provides shade, beauty, habitat, etc.

I wouldn't recommend slapping a bunch of oaks and maples in the Arizona desert, and I also wouldn't recommend covering South Carolina in Palo Verde.

Edit:

I also agree that there is an increasing need for energy efficient air conditioning that can be powered by renewable resources. I do not think vegetation is the only answer to this problem. We are at a stage where we need a multi-pronged approach to these vast and complicated issues.