How can the government have so many stats on crime but doesn't know how many crimes were committed by illegals?

Very good question!

Not true. The FBI is charged with collecting crime statistics for Part I Category crimes every year, and publishing those stats the following October: it's called the Uniform Crime Report. Those stats are EXTREMELY definitive and detailed. Three different times in my career, I was charged with the responsibility of reporting those stats for the department with which I worked. However, here's the kicker. There are NO statutory requirements for ANY law enforcement agency in the country to reports its crime stats to the FBI, period. They might do it for a few months a year or all twelve. They might do it for a few years, and then quit altogether. The point is that the stats ONLY reflect the criminal activity that is reported, and it's NOWHERE NEAR 100% of the crime in the country, simply because there is no legal compunction for law enforcement agencies to report. THAT SAID, the stats are still VERY voluminous because of all the departments that do report. And the report DOES collect data on the criminals committing those Part I crimes. The citizenship status of the perp(s) is reflected in that annual data.

Because the government can only track REPORTED crimes. So when illegals perpetrate upon illegals or citizens victimize illegals there is a good chance the victim if they are illegal will not be filing a police report. Just as tracking a Teflon-coated drone is difficult so is tracking activities within a community that decides to 'fly under the radar'--until an illegal is caught perpetrating on a citizen.