Basically because you have the US, Russia, China, and everybody else. A lot of the issue has to do with landmass and resources - again the same big three. Russia and China at odds is a problem for them because they share a common border (would be long and protracted or quick and nuclear), but still if you look at war with Russia and China it's like getting sucked in and devoured by a huge fluffy pillow. The suck you in deeper and deeper, then destroy your logistics, and have you piecemeal for lunch. The more Russia and China annex countries like Ukraine, the bigger the pillow gets and the worse for an invader. Look at WW2; Germany had it's way with most of Europe, couldn't handle Britain because of US support and because Britain had a lot of water protecting it, and Russia just absorbed what Germany sent at it. Russia moved factories out of reach and again like Britain had US support.