tl;dr version: the fear is that direct NATO intervention will lead to World War III. Biden's strategy takes a longer time to work. Russia and Ukraine will also need to consider supplies after this weekend.
There's also the hope that Ukraine will hold out longer than Afghanistan's government did last summer; at least with say Kiev holding out until late March and the Ukrainian government holding together along with a large conventional force until April/May. If they can do that, Russia's military would need to resupply (vulnerable to partisan and stay-behind operations) and occupy (there's plenty of people who wouldn't kill a soldier but would be willing to blow up a railroad, send videos clandestinely, host a nice polite young man without questions, etc.)
Likewise, Ukraine will need its own resupply, but the ports of Odessa and (AFAIK) Mariupol remain open, as do the land borders to Romania, Moldova (Transnistria might stop that), Poland, Hungary, and Slovakia.
Putin is driving for Kiev, and hoping to decapitate Ukraine in one blow and install some puppet to "surrender". From there he is hoping it will be like France after the fall of Paris in 1870 and 1940, with Ukraine meekly falling into line, with any resistance movement fading away through military defeat + mistakes on the part of the resistance + a global wish to just move on.
Because the alternative (for him) is 5x worse than Afghanistan - a protracted occupation which will end any impulse to end the sanctions and put Russia into South Africa in the 70s and 80s territory. Russian retaliation (not known for its restraint) and such will get immediate play worldwide.
He's also hoping that he is de facto forgiven as he was after Georgia in 2008 (Russia didn't have any major goal of actually going into Tbilisi and so Putin's aims were 100% achieved) and after Crimea in 2014 (Russia occupied regions that were 80-90% in the tank for Russia). But his aims at least so far seem broader than they were in Georgia and Crimea - his armies have progressed further into Ukraine - and in a few days will need resupply. This article talks about the challenges Russia faces in invading the Baltics, but the supply capabilities remain the same.
https://warontherocks.com/2021/11/feeding-the-bear-a-closer-look-at-russian-army-logistics/