My entire family has had identity information stolen from multiple places in the last 20 years.
My youngest daughter got a letter from the IRS about 6-7 years ago that someone had filed for employment and later income taxes using her information. The IRS knew 100% it was fraudulent, but they just sent us a letter letting us know it happened and that they knew it was fraudulent
Instead of sending me a letter that someone used my SSN to apply for employment and file taxes and you know it's fraudulent, go get them instead.
The IRS knows exactly where a ton of illegals are, they are using stolen SSNs for employment purposes. It isn't just in blue states
Companies know it too
I had a relative that worked in fast food, he said it was common for people that worked at the same restaurant to use the same SSN, the fast food chain never questioned it or did anything.