And given the relative weakness demonstrated by the west in recent history (such as abandoning Afghanistan) they might judge that in the end, the rest of world won't have the stomach for any costly intervention.
Sending the weapons to Ukraine amounts to little, in the face of overwhelming Russian military advantage. Given the likely goal of establishing pro-Russian government in Ukraine, and the long history of association, Russian forces are being rather "gentle" compared to what they demonstrated elsewhere, but if it comes to it... Ukraine cannot win without real support (as in boots on the ground) from the west, and nobody wants to contemplate that.