If it were the case that patents did not create monopolies, one would expect the same monopolies to develop in a market where patents are enforced to also develop, without exception, in a market where they are not. Monopolies of the sort that develop with the existence of enforced patents will not exist in a free market devoid of them.
The reason monopolies allowed by patents and other intellectual property could not be developed otherwise is one of scale. If you have a product that you hold the manufacturing process or code to and your product remains unpopular or of relatively low popularity, it is quite easy for you to keep it to yourself without government aid. If, though, your product is a real advancement that the market wants, you must expand your operations. This expense of keeping that process to yourself dramatically increases with scale and demand for that manufacturing process or code.