Presumably because that would push them into the hands of Russia (whether they want it or not).

Because Ukraine army salaries are paid by the government of Ukraine?

If Ukraine falls, Russian tanks will be withing zero kilometres from EU border, and four states who are only relatively recent NATO members, without much weight in the NATO block.

Add to these that all four of those states are far from being 100% reliable, with clepto-political elites who are very similar to ones who switched sides to Russia in the Ukraine conflict.

The nightmare scenarios is of course if Poland gives in. Then, you will only have 300-500km of very flat land, and good roads in between them, and Berlin, Vienna, Prague, Dresden, Nuremberg, Munich, and ability to cut off the Eastern route around alps.