Because they are, as a group, very child-like.
The primary reason for this is the abject failure of the public school system to instill any desire for critical thinking, paired with its failure to teach and celebrate our form of government.
Secondary is the desire of most people to do nothing and get paid for it (on someone else's back). This is part of being a child (children, after all, just ask Mommie & Daddy for money, or get an allowance, instead of working for it), and partly due to the lack of simple education regarding economics.
I have little but contempt for these people. I suppose that it is OK to be a bit juvenile into your mid-20s or so, until you get a job and get married and have kids. However, at that point it is time to grow up - but many people simple refuse, as that might take effort, particularly mental effort. They're just a bunch of overgrown children.
Another problem with liberals in particular and communists in general is to not understand how large systems operate.
If one were to focus on one “diasadvantaged” family and think “what would happen if the govt ‘helped them’”?
You’d come to the conclusion that the one family would benefit and if you’d divide the cost across the whole country, no one would notice the difference.
So a liberal would think “Well if it worked for one, why not do it for all?”.
The problem of course is if you do it for all, the whole thing breaks down bigtime.
But liberals are too stupid to see, realize, or be aware of the difference.