Because otherwise, after COL, you might only save $10k per year in a developed country, and $30k per year in a developing country.
That would be just as lopsided and unfair to your remote workers as paying them all market rate, so that your employees in developed countries get paid $80k per year and your employees in India get paid $8k per year.
Or do you expect your developers in America to relocate to India?
If you've identified the "best" candidate for a position and want to hire or retain that person, you're now competing with the other offers available to that candidate (which include CoL-adjusted offers), not with the salaries you're paying other employees.
Well, I think the idea is meant to be that you get the best developers possible, regardless of where they are, while still trying to keep expenses minimal.