Alice has an idea. Independently, Bob has the same idea. Neither one has ever talked to the other. Alice develops her idea and on January 1, files for a patent on it. Bob develops his idea and files for a patent one day later, on January 2. While their patents are being examined, both develop a product that incorporates the patented idea and both commercialize their product.
Alice, as the first inventor to file, receives the patent. And now Bob can’t sell his product without infringing Alice’s patent—even though he invented his idea without any knowledge of Alice’s work, and even though he came up with the idea at essentially the same time, and even though he might have been selling his product up until the day Alice’s patent granted without any legal issue. Bob’s an inventor who can’t use his own invention.
That’s how U.S. patent law currently works. It doesn’t matter if you came up with it without any help from anyone else—if there’s a patent, even if you had no idea, you’re liable for infringement.