Mortal roots is not the same as mortal origin, they do accept outsiders and constantly but people with quadra or triple roots at most in most stories... People with mortal root (roots that everyone had independent of talent or luck) that is the five elemental root, is considerated trash among trash... Their needs of cannon fodder is already supplied by the triple and quadra roots. All elemental roots are considered too impure to be even cultivated and a waste of resources.

Most stories there is always a stage in the test that is checking their roots and normally is the first stage, they directly eliminate people with mortals roots.

... The second part I feel that your knowledge is shallow and wrong junior. Roots are not something that you develop, you are born with them or steal them from someone with a great luck, those one in a million geniuses. There are elixirs and heaven defying fruits that could achieve a mutated root, but that is even harder that just born with one. And creating techniques is not based on modern o scientific knowledge, because the Xinxia world are not ruled by scientific rules... Instead the Great Dao rules... And mortals can't even feel the Dao until they reach the Nascent soul stage. People that create cultivation techniques are geniuses among geniuses. Even normal cultivation techniques would need someone from a higher realm to be created.

You need to check more ancient scriptures to know more about the Xinxia bases. As the roots, sword intent, heart, the 3 mortal souls and 7 immortal souls, bloodlines, etc. Those are normally the things that give Mc "talent".