They didn't. Standard Oil was cheaper, faster, easier, higher quality, more consistent.. why use anybody else? They weren't being manipulative, they weren't strongarming competition, they just made the best product and everybody wanted it. What's wrong with that?
I'm not really sure what is wrong with that, but the US Supreme Court seemed a bit troubled by it. There's a reasonably well cited summary on Wikipedia:
https://en.wikipedia.org/wiki/Standard_Oil_Co._of_New_Jersey...
In an adversarial court system, it’s worth keeping in mind that while the Court is who you need to convince, the Court is not your adversary.
On the off chance you’re not making a joke, the story is fascinating and far more nuanced than that. This is one of my favourite biographies and worth a read if you haven’t already!
Repeat ad nasuium for your favorite monopoly/duopoly.
Currently. one of the big dogs needs to make a big misstep in order for a smaller company to come in and win. Like car companies being complacent and Tesla seizing the opportunity. Electric cars had been around for a while, hobbyist putting them together but no capital to compete with Ford/GM. Now that Tesla has shown there is a path forward, look at all the capital that has gone to create new car companies.
Apple is not a phone or computer company. They are a luxury brand that is functional. Trying to compete with Apple is like trying to compete with Disney; you cant just be putting out a cartoon movie, you have to have the theme parks, the toys, the cruises. Maybe you can pull it off, but not very likely.