"Free market" isn't a particularly well defined concept given the broad range of its interpretation and use, eg: Churchill's "Land is the mother of all monopoly" comment wrt land ownership dragging down a truly 'free' market., etc.
Standard Oil wasn't a monopoly either. There were other oil companies in the business. But Standard Oil was selling oil, at a profit, at lower prices and in greater quantities than any of its competitors. The competitors didn't like that and got the government to break up Standard Oil under antitrust law. Which, as I've said, made things worse for customers.
In other words, if the competition simply doesn't compete very well, or makes bonehead decisions, a company can get a large market share, yes. But "large market share" is not the same as "monopoly".
If you want a better example of government created monopoly, look at ISPs in many areas of the US, where for many years state and local governments gave one company exclusive rights to sell Internet access in particular areas. With the expected result that Internet access sucked as compared to areas where there was competition. Much of that has gone away now, and Internet access as a result is much improved in those areas.
And then prosecuted them under US antitrust legislation?
Standard Oil Company and the American Tobacco Company were also US Government monopolies?
That sound a little off.
Perhaps your use of "almost always" needs to be dialed back a notch .. or a lot.