Because there hasn't been any credible threat for decades.
But now that Russia is back as an evil "superpower", defence spending is sharply rising in Europe.
And so far there is no real indication that Russia will ever take the Ukraine let alone attack the EU/NATO. If Russia or China were to directly attack NATO, you'd see defense spending going up pretty sharply.
But even now, nobody believes that Russia or China will dare to attack NATO in the near future. It probably will be limited to a proxy war (proxy for the NATO side) in the Ukraine and/or Taiwan.