If 10% of engineers in your country end up working for a foreign firm, wont SE make more money and have less costs vetting remote postings?

There's plenty of opportunities they left on the floor because they didn't like the best deals of that business, sure they could have grown and lowered their average return.

US companies aren't really interested in selling their services at such a low price. They have US salaries to pay.

Then why they are outsource driven from famous Asian consultancies?

500$ for post on single site. And there is probably half-dozen of them around... That really starts to add up in long run.

$500 is a very low estimate of how much value a company steals from society every time it adds an additional posting to get people into its permanent hiring funnel and uses them more to tune that funnel than for serious consideration. Interview 10 candidates and pick one or demonstrate that you at least are willing to pay 1/10th the costs of your folly.