Because LLM-created content is not an expression of your own human creativity or intellect.
It's not like typewriters -- in a written work the content is the entire point, not the handwriting. So unlike previous tools, this one is replacing you for the part that actually matters.
People use these tools for a variety of reasons (as diverse as people’s experiences). One can use an LLM to help express a perspective or develop and opinion (very important for those who struggle to communicate), or one can fake a picture or voice for fraud, or a million other purposes. It’s just a tool. How it gets used is about the people, not the tool.