> That presumes Google, MSFT, etc. cannot or would not be able to find an equivalently talented American.
Multinational product companies are not really constrained by geographies. Once somebody passes the interview bar and is deemed desirable, a hiring process kicks in to solidify that hire. Sometimes the specific geographic office is critical (small and concentrated team), sometimes they're hiring into generic "software engineer" position, where personal preferences and ease of visa procurement decide the specific office of employment.
International outsourcing companies, however, are very geography-constrained and skills-oriented, as their needs are driven by clients. So if they are looking for someone with 10 years of Oracle RDBMS and specific integration experience with SAP, Siebel and Microsoft Dynamics back-ends in Kansas City, the skills most of the time are very specific and the employment must occur in Kansas City at client headquarters.
> there are few and far between advanced economies promoting the benefits you speak of in their own counties
Ireland and UK have fast-track immigration procedures for technology employees and buy ads in technology magazines claiming they're "open for business". Canada bases their entire immigration system on education, qualifications and employment availability where someone with technology skills and an employment offer is almost guaranteed to gain Canadian residency.
Some other countries do not have revenue dependence on personal income taxes, and thus couldn't care less if employees within their jurisdiction earn large salaries. Others derive majority of the revenues through natural resource exploitation with entire economies based around those industries.