.....And this is the gist of my reply.
The main reason is because there is very little profit in combating the, self-styled, Islamic State (IS, formerly ISIS/ISIL). And NATO is primarily a money racket which provides a lot of influential people with a very comfortable lifestyle, flying around the globe on private jets and generating well-paid jobs, both directly and via the think-tank circuit.
Let’s be clear here. You don’t require submarines or nuclear weapons to engage ISIL, but you would in a putative conflict with Russia. The fact that such a collision would probably, due to the atomic arsenals involved, mean the end of human civilization, is irrelevant to NATO because it doesn’t actually want a war with Russia. Instead it desires to use Moscow as a convenient bogeyman in order to maintain US defense spending, which grew 9 percent annually from 2000-2009 during the “war on terror.”
That massive increase in expenditure was justified by the need to remove, initially, Osama Bin Laden and, later, Saddam Hussein from the international stage. After both were eliminated, in reverse order, you'd have expected the splurge to be cancelled and for the largesse to be reduced to pre-9/11 levels.......