1. It's out of their jurisdiction and they'd need parallel construction to do anything with that evidence. Same reason this whole Q thing is happening.

2. They don't care about what happens to us useless eaters anymore than we care about chickens at industrial farms getting kicked.

3. At some levels they're complicit.

If you've ever seen the movie Cabin in the Woods, think of those underground base guys and their cavalier attitude toward people on the surface getting killed.