There are many arguments that socialism doesn't work. It's becoming an increasingly popular topic as congresspeople such as Bernie Sanders and Alexandria Ocasio-Cortez push for policies that are considered to be socialist.
However, the articles about why socialism fails generally cite multiple different reasons, and there seems to be no clear argument.
The only reason why something would fail practically when it should work theoretically is that it is making incorrect assumptions about the world.
What assumptions does socialism make in order to work theoretically, and why aren't those assumptions accurate reflections of the world?
(Please note that this question takes the standpoint that there are incorrect assumptions in the theoretical basis of socialism, and is trying to identify weak points in this theoretical basis. It may be true that socialism does work, or that there's a version of socialism that works - this question is specifically asking about what parts might not work. Similar questions may be asked about capitalism, or other economic theories.)