I'm not sure on what basis you're claiming that free markets prevent monopolies, but even if they do, free markets self-destruct. Once corporations in a free market have enough money to start influencing politics they use it to start regulating in their favor and the market ceases to be free. You can regulate to prevent that, but then the market isn't free, yet again.
It's also a bit myopic to view companies as a binary monopoly/not monopoly. A company with 90% market share isn't a monopoly, but has most of the downsides of a monopoly, and likely is a monopoly within subsets of the market.
It's also a bit myopic to view companies as a binary monopoly/not monopoly. A company with 90% market share isn't a monopoly, but has most of the downsides of a monopoly, and likely is a monopoly within subsets of the market.