There is a lot of inefficieny in the market, some of it inherent, some of it due to artifical constraints like location, but over time those tend to naturally get normalized. What I'm trying to say is that if it really were better for software to be built in Vietnam for 10% of the cost, that would just happen.

It may be that companies have not figured out this neat trick or it might be that there are some inherent issues with developing software based purely on the cost.