Because it's not natural. It's not natural for people to spend roughly a third of their adulthood doing a job they hate to get money which which they can satisfy their needs, if that, so they can continue doing that job they hate.
It's not natural for people to destroy the environment and wildlife to build and live in large cities (which are nothing more but concrete jungles which, sooner or later, become concrete deserts).
It's not natural for people to wash off the fertile soil into the sea with intense and large-scale agriculture.
It's not natural for there to be large factories which exist purely to satisfy the consumer's needs for goods, disregarding all the consequences which may come from that process.
Nobody would do this, unless forced. That's where slavery comes in. If you look at history, all the great civilizations practiced slavery. Mesopotamians practiced slavery. Babylonians practiced slavery. Sumerians practiced slavery. Egyptians practiced slavery. Jews practiced slavery. Greeks practiced slavery. Romans practiced slavery. Aztecs practiced slavery. Mayans practiced slavery. Etc, etc, etc.
You may say "But slavery doesn't exist today". Except it does, and at large. The only difference is the intensity of enslavement and the people's lack of awareness of it. The governments all over the world are modern slavers. They have strong police and military forces which will make you do as they say, whether you like it or not. That's slavery. In the words of Mao Zedong: "All political power comes from a barrel of a gun".
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