Lot of reasons. Genetics plays a big role in it. You can get seeds like you talk about and if you have time money and patience slowly refine the strain to its best aviliable output. You would grow a batch of seeds at least 20 or so get them through veg and then get the strongest fastest fullest females and then clone those then as the clones go keep the best of thbest as moms showing the best triats. Work the grow with good nutes care and light.Find which clone gives the best new plants in flower then you know of all the original cloned mom who is the best grower producer and highest thc content and there is your mom. May tak e 6 or 7 grows to weed through the weed to find the right 1 but thats how its done.