FTA: Citizenship and Immigration said in its report that coming up with a number for H-1Bs living in the U.S. “is complex, as no electronic data system tracks or houses this information.”
Strictly speaking that's insufficient as crossing from the US into Canada by land doesn't invalidate your I-94. They do need to join it against a bunch of other data -- which they do have.
Exactly. They have the data, but they’d need to link it to a few other sources for it to be complete.
The US, unlike Europe (at least in my experience) doesn’t check passports on the way out. They rely on flight manifests or information from Canadian authorities on who has exited.
When you enter the country on an H1B or otherwise do anything related to any visa status, the onus is on you to carry around all relevant and supporting documentation, in physical, original paper form. Visas, passports etc. existed before computers, probably in much the same way as they do today.
Every single foreigner is assigned a unique alien number and tracks your entry and exit into the country.
They just don't want to deal with connecting information from two different departments.