In order for something built to stand for a long time, it must be built on a solid foundation. While you can just pour a concrete slab for a house on dirt (houses in the south don't have basements), if you want to build a skyscraper, you have to drive pylons deep into the ground until you hit bedrock and then build from there. A skyscraper built upon a poured slab on dirt will topple over before it's completed.
Jesus spoke on this in Matthew 7:24-27: “Everyone then who hears these words of mine and does them will be like a wise man who built his house on the rock. And the rain fell, and the floods came, and the winds blew and beat on that house, but it did not fall, because it had been founded on the rock. And everyone who hears these words of mine and does not do them will be like a foolish man who built his house on the sand. And the rain fell, and the floods came, and the winds blew and beat against that house, and it fell, and great was the fall of it.”
It is the same with the Obama legacy.
The Trans-Pacific Partnership, The Paris Agreement, DACA, the Iran "deal" and more have all been, or are in the process of being nullified precisely because Obama did not lay the proper foundation. In each of these cases, Obama did all these using only "his pen and a phone." The Senate did not ratify any of these agreements, nor was there any legislation passed by the Congress to show assent by the Legislative Branch for these actions. Trump is now dismantling all of these "Obama Legacy" achievements and more piece-by-piece simply because there was no foundation that these "cornerstones" of Obama's were built upon to they could stand in the face of a president that doesn't share Obama's ideological beliefs.
This, more than anything shows Obama's arrogance, hubris and exactly how good of a "Constitutional Scholar" he really was. If Obama really were the "Constitutional Scholar" he claimed he was, he would have known that any future president could by the stroke of their pen, nullify all of his "great achievements."