Because that's how the world works, one single persons says No and history is written, the world changes.

Russia entered the Ukrainian civil war precisely because it sees NATO's encirclement as an existential threat. As such, Russia will not cease until its strategic goals are met. Kiev could end the war as well by ceding the Donbass region and agreeing to NATO neutrality. The Donbass region already is more pro-Russia than pro-Ukraine, even if Kiev succeeded in pushing Russia back to the pre-2022 borders, they'd still have to contend with the seperatists they have been fighting since 2014, and failed to contain. Cozying up to NATO and agreeing to host missiles that target Russian cities, with less than a 10 minute travel time, is precisely what caused Russia to shift from diplomacy to active intervention.