Why would the NATO not defend its members against Russia?
Lack of political will basically (for which one can find many reasons or justifications, of course). Each NATO country can choose how to respond to an article 5 invocation. This is why Trump's discourse (about maybe not defending a member) worries people. After a bit of digging, the promises in art. 5 exist in that guarded form supposedly because:
It is important to recall that the U.S. Executive
Branch, which was responsible for negotiating the
Treaty, insisted on qualified language in Article 5
largely to assuage concerns in Congress. Congress,
which has authority under the Constitution to declare
war, did not want to cede that power to any
And as a result, any country which depends on its parliament for going to war (this also includes Germany), can say "no thanks" to any NATO military action. Which basically means, to twist a term that any actual NATO defense is done by a [sub]alliance of the willing... (I'm not sure what can happen if Congress decides to go to war but the US President opposes it; that's probably worth a separate question.) By the way, article 42(7) of TEU is basically in the same boat as art 5 of NATO as far as its implications for obligation go. The advocates that wanted stronger promises to be made in TEU ran into the same problem with some countries wanting to have the option of staying aside in any collective defense action.
However, simply seeing it like this, ignores how NATO is actually perceived by (most of) its own members, e.g.:
Q. How do you get countries on the eastern flank to devote resources to the southern flank, when Russia is almost literally breathing down their necks?
A. It’s easier than you would think. Because every ally understands that the most important value of NATO is unity. If we don’t hang together, we will hang separately and so while Eastern allies are very understandably heavily focused on Russia they absolutely get it — that the alliance including they, themselves, have to contribute to concerns southern allies have. And the same is true the other way around, southern allies are absolutely engaged on challenges to NATO [from the east.]
So basically this is a big part of backbone of collective defense: if you chicken out in helping someone... there's a good chance they'll pay back the same. Which is probably why the US is the one making most of these not-gonna-help noises: they need other countries the least to defend their interests, or at least Trump thinks that. In particular since in the case of Russia as enemy...
The decision to act, or not, would be made not at NATO HQ in Brussels, but in Washington, DC. And, many eastern NATO members worry, it is hard to imagine an American president risking nuclear war to defend a tiny country half a world away.
Besides how NATO countries would actually react, it's also important to consider what its potential enemies think NATO countries will do if one is attacked:
Article 5 says that the response may include armed force, but it does not mandate it. All that NATO actually promises is to take “such action as it deems necessary” to restore and maintain security. That could be anything from nuclear war to a stiff diplomatic protest. Three tricky considerations would determine the precise nature of any NATO response to foreign aggression. The first is geography: in places where an aggressor can quickly complete and consolidate an invasion, NATO's options are very limited. The Baltics, for instance, occupy a thin flat strip of land which is all but indefensible. A Russian surprise attack could reach the coast within hours, and reversing a successful Russian invasion would be hard, even futile. Yet that was also true of West Berlin. The Baltics argue that an attack on them would mean an all-out East-West confrontation thanks to Article 5. If Russia believes that, deterrence is working. But Article 5 does not specify such a response.
That it comes down to: perception is reality in this case. It doesn't matter that the treaty says NATO countries can do nothing... if Russia thinks that's not the likely response to its aggression. That's why many NATO people worry about Trump busting this perception in eyes of the world.
And finally, there's another aspect consider: ongoing military cooperation, besides enhanced cooperative readiness, also gives a potential "Pearl Harbour effect" (if I may coin a term) for US troops stationed near Russia's border:
Another option would be to put American soldiers in harm's way, so that Russia could not invade NATO territory without directly harming American military personnel. Given that America could not help but respond forcefully to an attack on its own people, such a move might render the NATO guarantee toothy. Not toothless. So, basically, man the NATO-Russian border with American troops.
It doesn't have that close, but you can see that having to dispose of some American targets (e.g. air power or even just AWACS, or anti-ballistic missiles) in Eastern Europe could be a big problem for Russia if it has the potential to create a "Pearl Harbor effect" (albeit not on US soil) that could enrage the US public and the Congress. Political will can materialize surprisingly quickly in such a case; even a just a proportionate US response could mean trouble for Russia as an aggressor in this case.
So my point with this is that while on paper the NATO treaty may look weak, the deterrence effect can well exceed what simple reading of the text might conclude... as long as the magic is not busted by talk to the contrary.