Because the only method shown to prevent war in Europe is co-dependence. EU is founded on integrating the belligerents of WW2 close enough that it would be unthinkable to happen again.
The same method, which at least has worked previously, was tried with Russia and now 30 years later, or more likely in 2014 with the invasion of Crimea we can conclude that Russia is different in some core way of thinking. Not too different though, looking at how the Baltic states have developed when given the choice and chance.
The alternative scenario, with a Russia still riding on the tail ends of soviet military might in steep decline not even given a chance in the early 2000s seems like a recipe for an even worse outcome than we have today.