Because the total size of the Ukraine armed forces of allservekces is smaller than the Russian + Belarusian land forces deployed against them, not to count the supporting air and naval forces, where Ukraine is outnumbered by more than an order of magnitude.
> Unless a major power steps in to help ukraine, it'll be "liberated" in the next few weeks. And then ukranians will go on with their lives.
This is as stupid as when people said the same basic thing about the 2003 Iraq invasion. (Something along that line is, IIRC, a big factor in Gen. Franks describing then-Undersecretary of Defense for Policy Doug Feith as “the fucking stupidest guy on the face of the earth”.) People can't be relied on to just go back to their lives when a hostile foreign occupation is imposed, even if the regular forces of the preceding government are comprehensively defeated.
> No country is going to step in to fight russia.
Just like no country stepped in to fight the USSR (or the US) in Afghanistan. And neither of the armies involved in that had the degree of morale and logistical problems the Russian Army has displayed thus far in Ukraine.