Because there’s no law against copying an *idea*.
Patents only protect *inventions*. While the law is far from perfect, the idea of patent law is that it’s supposed to provide limited protection for a truly novel invention that would not be obvious to someone else skilled in that particular field.
So having the idea of a vacuum cleaner isn’t patentable.
A specific vacuum cleaner that uses one particular mechanism to create suction might be patentable (and lots of patents have been granted on vacuum cleaners). But that doesn’t stop a competitor from inventing a different mechanism that achieves the same effect.
Lots and lots of products don’t even require inventions. If anyone else would have been able to easily come up with basically the same product once they hear about the idea, then it’s not patentable, and there’s nothing stopping you from copying it.
The other forms of intellectual property protection are trademarks (you can’t copy another product’s name), and copyright (you can’t copy another product’s text, the exact shape, the exact color/design, etc.).
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