Imagine if you could outsource all your creative work (Assuming you're not a creator and would be outsourcing this work anyway) to a single person. There's this one guy who is amazingly prolific, everybody loves him because he's extremely reliable, always turns in assignments on time, never complains about reverts or edits, and is breathtakingly cheap.
The only problem with this guy is that he's writing for all your competitors. So all your copy, content, advertising, slogans, designs, graphics, etc... it all looks like it was done by the same guy because it WAS all done by the same guy. And he tries to mitigate this by offering different styles of writing or drawing, but at the end of the day people start to recognise his take on these styles and so it all becomes samey.
You lose your creative edge. You lose your distinctive look. You and your competitors all start to look the same.
I'm talking about generative AI, obviously, but we've had earlier versions of this problem for ages now. Small businesses not bothering with a website because a Facebook page is free, even if it allows zero customization and your facebook page is identical to your competitors, and both look like a Facebook page, not a Your Business page. Or maybe you even have a real website, but... templates are easier and cheaper and faster than hiring a real creative designer to create a unique design from scratch. So, again, you look exactly like everybody else, only you painted your text background #FDFDFD and they went with #FEFCFC