The entire point is that we do NOT live in a post-scarcity society. The real world is not post-scarcity. We do not live in Star Trek universe where you have unlimited anti-matter energy and can just shove a tree or something into a matter replicator and pop out a steak dinner.
There is currently a minimum required effort to survive, even if that effort is "plant seeds, wait, harvest food."
What is the incentive for the farmer with ten cows to collect the milk and give it to people in his community for free? You could theoretically say something like "mutual protection from other communities," but then you need to ask "what happens when even a single person in this proposed community doesn't bear their own weight/responsibility for community protection/prosperity?"
For reference, look up what happened to the original Jamestown colony in America. I'll give you the run down: colonists had communal food storage. Colonists didnt contribute to communal food storage because lol everyone else will provide the food. 90% of them starved to death the first winter. Captain James Smith (of Pocahontas fame) shows up the next spring and says "ok, fuck this shit, you don't work, you don't eat," and lo and behold, they all work to provide food and goods, and have a basis for barter/trade (you give me milk, I'll give you this table I made, i.e. capitalism), and shit improves.
Socialism instantly and irrevocably fails if even a single person doesn't bear their own weight, and if your basic survival necessities are provided for, you have zero incentive to contribute to the well-being of your community.
Socialism sounds great until you realize that other people are dicks and don't have your best interests at heart; it is literally hard-wired into our biology to compete for resources since resources will always be limited in our current reality.
When scientists invent a way to turn a pile of dirt into a steak dinner in five seconds at the push of a button, then I'll support socialism. Until then, you're a potential competitor to my resources, and you can fug off unless you can provide me equal or greater resources in exchange for mine.