Imagine you just didn't invest those millions of dollars, then, and instead someone else developed the idea and didn't want to freeze the rest of the world out of using it.

Patents only makes sense if you assume that the alternative to you inventing something is no one inventing it. Experience shows that's very rarely the case; in general, when an idea's time has come (the base knowledge is there to understand it, the infrastructure is in place to use it effectively, etc.), there is a race between many parties to develop the idea. Applies to everything from machine learning models to the light bulb or telephone, both of which were famously being developed by multiple inventors simultaneously before one person got lucky, often by a matter of mere days, and was issued an exclusive license to the invention, while everyone else who had the same idea was out of luck.