If the best talent or opportunity isn't in the southeast, then companies won't go there, so I think it's not as simple as "just open your office in the southeast". Companies are distributed closer to their relevant hubs. Like you said, people don't want to move, especially if it means taking loss in efficacy.
It's not about the capability, it's about the switching cost. A business's cash flow is like their blood flow; they do things that bankrupt them, they die. You can't expect businesses to go bankrupt to perform a contrived move. There is an energy cost for every change.
How many major tech companies have serious hubs in Atlanta? Some of them, but certainly not all.
People don’t like to move. A nontrivial amount of the pipeline problem can be addressed by opening offices in the southeast.