There should be, except if you plant trees they get in the way of parades and fireworks viewing and fall on guests in windy conditions. They have to make choices between shade and views and safety and it will never be a perfect situation.
I always think about the trees Magic Kingdom had-- beautiful shade trees in the Hub and even along Main Street. They were all removed so people could see the fireworks and projections and had more parade viewing space. As a result of the lack of shade the park sells more $5,6,7,8,10 drinks.