The key to the answer lies in the story of creation. Hashem commanded that “eitz pri” (Braishis 1:11) appear – trees that were fruit, meaning the bark of the fruit tree was supposed to be as edible as the fruit itself. However, the trees did not do that. Instead, out of the ground came “eitz oseh pri,” trees that produced fruit, but were themselves inedible.
Rav Kook (Orot Teshuvah ch 6) explains the Midrash as a metaphor. Fruit is the desired end; the tree is the means of producing it. Ideally, the means to a goal should themselves have some inherent value. Reality, however, does not live up to the ideal. We perceive our stuggles toward various goals as obstacles to be overcome rather than as valuable experiences and endeavors in their own right. (Gush's VBM has a nice shiur on this here.)
If the trees did not fulfill the command of Hashem, why does the Torah says “Va’yehi kein” on that third day of creation? The Nezer haKodesh explains that it was only the trees of chutz la’aretz which disobeyed; the trees of Eretz Yisrael did exactly as Hashem commanded.
In light of Rav Kook’s metaphor, what this means is that in chutz la’aretz there is no inherent value to farming, building, etc. These are just means to an end, and if we could skip the intermediate steps and get what we wanted some other way, all the better. Not so in Eretz Yisrael. There, ends and means are one and the same; the tree is as tasty as the fruit it produces. The process of building the land is valuable in its own right and deserving of reward.
Now we can understand, explains R' Ya'akov Moshe Charlap, the great student of Rav Kook, what Moshe wanted. “Do you see trees, like you do in chutz la’aretz”, he asked the spies, “Or are you able to see that Eretz Yisrael is a land of fruit alone?” Moshe was challenging the spies to report that in Eretz Yisrael the ta’am ha’eitz is the ta’am ha’pri, it’s all edible, everything has value, there is no difference between means and ends. Eretz Yisrael fulfills the ideal that Hashem commanded for creation.
Had Moshe brought us into Eretz Yisrael, this is what we would have seen. And, says R’ Charlap, is we are zocheh, we can and will find the trees like this in Eretz Yisrael – trees that are really fruit, ta’am ha’eitz k'ta’am ha’pri, just as Hashem commanded.