If the Russian government believes it can just roll tanks into any neighbouring country as it fancies, cheaply and quickly, confident that the citizens of that country will surrender happily and have their freedoms taken away from them, they will start more wars and claim more territory.
In war, there are mainly losers. It's true Ukraine cannot win this war, but at this point, nor can Russia. Russia doesn't even have enough soldiers to hold the entire country of Ukraine at this point, and maintaining control over any territory it captures will be expensive. That's why they are focussing on capturing the Donbas region so they can claim victory and gain territory without losing face.
We have to take into consideration that if Russia did annex Ukraine, it would likely lead to extreme amounts of sufferring anyway - possibly including civil war, insurgency etc. that could go on for decades.