I suspect there are many reasons I'm unaware of, but two things come to mind.
First, the IRS is chronically underfunded because their budget is approved by Congress, and Congressman get funding / donations / bribes by people that don't want the IRS going after them for their shenanigans.
Second, there is (understandably) controversial book by Bruce Gibney called "A Generation of Sociopaths: How the Baby Boomers Betrayed America"
with a recent interview here
"They habitually cut their own taxes and borrow money without any concern for future burdens."
The general mindset seems to be to kick the can down the road, on all things, including the environment. This group became the dominant political power in the mid 90s, and still holds great power.
Sort of off topic, but I do agree that the older generations in the western world, particularly the american baby boomers, act sociopathic as a group. There is a good book about this: A generation of sociopaths. 
Sad to watch an entire generation hate their children and grandchildren (if they even have them). Believe me, I do not enjoy painting an entire group of people with a shit-stained brush, but it is what it is and it should be called out.