Because it is expecting human beings to all think a like and agree to have equal everything, which has never ever happened and won't as long as human's are on this earth. It becomes authoritarian (don't think simple governments) because without a controlling group it would never work. Hence a controlling power somewhere, which will lead to exactly what we have seen every where it has been tried.

Prime example was the CHAZ zone. It started off all love and drugs and free food, still it had a wall around it, self proclaimed militia guarding the entrances, over time the food ran out and people were killed when the same armed guards decided to start robbing people, people died!

Your utopia of humans all getting along, giving up everything for the common good, everybody has the same stake in everything is impossible. Go try it, get a group of likeminded people, give away every single item you own, start from scratch and see how long the group lasts without it either collapsing completely or one group taking over. It's the control of that group (example government) that needs controlling not the people of the land.

Here's another example of socialism eating itself. I watched some of the socialists of America conference back 2018. An issue arose where clapping or applauding was a perceived problem, it may trigger some people sensitive to clapping, so a new rule was voted for, that instead of clapping the audience should wave their hands in the air, so as to not "trigger" and (somehow) scare a couple of people in the audience. So they started waving, until someone said excuse me, what about the blind people!

You see with socialism/communism it is alway always fails.