Because most loopholes are in fact deductions that are there for a reason.

The overall tax rate is too high (or it would be too expensive to lower the overall rate), so politicians give certain people sweetheart deals on certain things because they consider it relevant, thus offering someone, somewhere a tax break. Others are then free to use the same deduction for their benefit.

The wealthy aren't stupid. They're generally quite pragmatic and flexible when it comes to making money.