Not just patents: trademark too.

There’s only one Facebook. Without that, you also don’t have Facebook’s network effect tying it all together.

The problem is that even if there are alternative ways, it's not always profitable to find them. One clear example is drug prices. The price to manufacture a drug is tiny compared to the cost to develop a new one, so of someone has a patent on a drug that cures a disease, it's not worth anyone else's time to develop a different drug. Getting your drug approved will take years (for good reason), and at the end, you have a product that competes with another company, where instead you could just develop a different drug and get your own monopoly. As such, there are product monopolies on pretty much all drugs to treat rare diseases.