More likely than not, it is because no matter how nasty and illegal you want to think hiding those assets off shore in havens might be, there is a possibility that it is done legally even if not ethically.
Look at it this way, several years ago, I purchased a rehab home. I moved into it, rented the old home out which made it my primary residence. I then used a grant through the utility company paid for by the state in order to remove the old plaster and lath, install modern insulation, seal the walls and drywall them. I had to pay for the ceilings but the rest came from the state through the utility company. But I also replaced the single pain windows with a tax program the state had so the difference in the trim actually allowed the purchase of the drywall on the ceilings. So essentially, I gutted and insulated the entire home, upgraded the windows for free if you accept spending money I would have already paid in taxes as not being a new expense.
I then replaced the aging roof shingles (slate) with Owens Corning 25 year shingles under another program for energy efficiency by the feds granted to the state. Because I went from slate to asphalt shingles, I had to install a venting system and all which ended costing me about $3000 but everything else was covered by the program. But there is more, I bought new appliances too with another energy grant through the utility company which was through the state so I ended up only paying 40% of the cost of those.
Basically, by the time it was all said and done, I spend about 5 years of my taxes on an investment home instead of paying the state and federal governments and got a bunch of free (think refunds) money on top of what I would have paid in. It was all legal as long as I lived in the house another 5 years. I sold it off at the height of the real estate bubble and made a killing. This was a tax shelter much the same way as those off shore havens work. It's all legal even though you think they shouldn't be.
The tax code is so complex that even IRS officials cannot follow it enough to stay out of trouble with their own taxes. What is obvious isn't always illegal and what is illegal may only be illegal in certain situations. And what is obviously illegal can be perfectly legal if your accountant structures something specifically with the intent of making it legal. Then again, people have been wrong before and get busted for thinking their illegal thing was legal but it turned out to be illegal. If that confused you a bit, it was supposed to just like the tax code.