Because, despite the fact that it has been a colossal failure everywhere it has been tried, Dems (in particular) and politicians (in general) are arrogant and believe that they can do it better.
What they overlook is that socialism completely ignores human nature. People need to own things and be in command of their own destinies. We define ourselves by what we have and what we do. These attributes are sources of pride that we express our individuality and uniqueness with. It is also the means by which we express the fruits of our labor. Socialism ignores ALL of these things and focuses solely on trying to make all people the same.
Unfortunately for the socialists, that's just not how God planned things and it's why it doesn't and will never work!!
The dopes think the reason it never works is because they were not in charge of it I guess.