Because every single time we do, it comes back to bite us in the butt!
Are you suggesting that bringing down the WTC towers was the price of bringing down the Berlin Wall?
Perhaps, it was.
Which, in the total scheme of things, probably means it was a good trade...
That's a stretch !
A case could be made that helping out the Mujahadeen in Afghanistan is what bit us in the butt. Training up a bunch of Sunnis in Iran and handing them a mess of hardware would almost certainly cause a butt-bite that will leave a mark.