The cost of enforcing extra-border tax law vastly exceeds the revenue recuperated from such an activity:

yield at best […] less than $1 billion per year […] while inflicting compliance costs worldwide of $1 to2 trillion

which begs the question, why bother aggressively pursuing / leveraging international intergovernmental relationships regarding resident/citizen finances?

Foreign account information is too important to a wide range of civil and criminal law enforcement andnational securityefforts

Of course.