Because one of the reasons we have a special day of celebration on Tu B’shvat for planting trees and not bushes, is because the reward for planting a tree takes many, many years to come about.
It takes a lot of patiences and delayed gratification to keep planting now, for a distant future.
The Talmud tells a story about a teacher who was not a very scholarly teacher, nor a very interesting teacher; but he was, like a tree planter on Tu B’shvat, a very diligent and patient teacher.
Rabbi P’rida had a pupil who had to be taught a lesson four hundred times before the student could master it. One day Rabbi P’rida was requested to participate in a mitzvah [after he taught this student].
He taught the student slowly and carefully [as usual] but the student could not grasp the lesson. Rabbi P’rida said to him, “What is the matter today?”
The student said to him, “From the moment the Rabbi was told that there was a religious matter to be attended to I could not concentrate my thoughts, for at every moment I imagined, now the Rabbi will leave, now the Rabbi will leave.”
Rabbi P’rida] said to him, “Give me your attention and I will teach you all over again”; and so he taught him another four hundred times.
A Bat Kol [a Heavenly Voice] issued forth from above asking him, “Do you prefer that four hundred years shall be added to your life, or that you and all your generation shall be privileged to have a share in the world to come?”
Rabbi P’rida said, “That I and all my generation shall be privileged to have a share in the world to come.”
God said, “Let him have both this and that.” (B. Eruvin 54b)