Because their economy would implode immediately if they did.
They’re already on the knife edge of mass open rebellion as it is.
Russia is not the big bad bear of yester year. Who gives a **** what Russia does other than the small nation states of former USSR along the borders of Russia.
What are they gonna do? Nuke us? Like that’s a win idea. “Hey! Lets all murder ourselves with nuke retaliation by the US!”