You have it backwards. They do not report data, because they choose not to collect it.
One anecdotal example from my personal life. After college, two of my friends and I moved into a condo in a low-rent part of town (we were still broke waiters and booksellers). We were the only white kids in about a square mile, the rest was all hispanic. Most were probably varying shades of legality, but we thought it was cool because, bloc parties and great mexican food.
One of my buddies had a classic mustang he had rebuilt with his dad - beautiful car. The guy had a kind of awkward relationship with his dad (his dad was kind of a Man's Man who thought men should make things. My buddy was a theater major who loved fantasy novels), and, I think building that car together was kind of The Thing They Had.
Anywho, he's sitting at a stop sign one day and one of our neighbors T-Bones him, doing maybe about 35, which isn't terribly fast, until it hits you in the side. The other driver then pulled back, and drove off. Car was all smashed up. Naturally, we called the cops. They came and took notes and then asked if we knew anything about the guy, a tag number, etc.
"Oh yes" we replied, naively, "he lives in that apartment section right there, and works at [Restaurant name] down the road - he's probably there now on shift, and will be back tonight."
The two cops look at each other. "Er, is he an illegal?"
"Uh, maybe - why?"
...And that is when we found out that the regular law that applies to you and me doesn't apply to illegals. Because it's too much trouble and paperwork, so long as they aren't actually physically attacking anyone, the cops were under direction not to ticket them, not to arrest them, not to take them in, not to do anything that would force them to notice that the people they were interacting with were here illegally. Tough luck, kid, you understand, I'm sorry, but there's nothing we can do - you'll just have to fix the car.
This wasn't in some kind of deep-blue San-Fransisco-type area of a deep-blue state. This was in Alabama. During the Bush Administration.
My buddy tried to confront the guy and get him to pay for the damages, but that went south pretty fast, and we left after being threatened with violence. We slept with loaded weapons next to our beds after that until the lease was up.
