How do you know "empirically" that they aren't? Who says that the US-sourced guns that they are tracing are even a substantial fraction of the overall guns in use? How can you prove empirically that the data provided by the notoriously-reliable and agenda-less Mexican government is accurate?
Mexico, 10 years ago: https://www.vice.com/en/article/the-cartel-gunsmiths/
Philippines, 13 years ago: https://www.youtube.com/watch?v=67PYuGQM9Fg
> Who says that the US-sourced guns that they are tracing are even a substantial fraction of the overall guns in use?
Statistics?
Believing your implication that homemade firearms or widespread and just don't show up in the seizure data is a little silly unless you can explain why this would be the case.