Apple makes it hard, if not impossible, to leave their ecosystem. And they are a large enough entity (in the USA where this is based) to use and abuse this size and influence.

With Android, if i don't like Samsung, i can use Google, Motorola, etc. Same with Windows. I can buy Microsoft Surface devices, or Dell, HP, etc. I could also buy a Linux computer, Chromebook, Macbook, Android tablet, iPad, etc...

With iOS, it's Apple or nothing. This makes Apple a gatekeeper, and a dangerously strong one. For more and more people, their "computer" is their smartphone, and with iOS, Apple is the first and final word. Want to sell to an Apple user digitally, Apple must get a cut, and they must approve of it. This just further empowers Apple, as they have more and more power and sway into what can and can't be done. Combined with the fact as I mentioned that Apple makes it so hard to leave them. It is this size and power that makes them a monopoly, not their popularity. It's the same thing that happened to Microsoft with it's monopoly lawsuits. It wasn't they were "just popular", its they used their size and power to limit/surpress competition to empower/enrich themselves.

As for your example of "better like Microsoft", I do have options, and those options will still allow me to leave them easily and non-painfully, or at least if I still use Windows I can use apps regardless of what Microsoft thinks or giving them more money "just because". Apple iOS doesn't allow this. As for power companies, ISP, streaming options, these aren't the same subjects so I'm not going to address them. And before it comes up, no, consoles aren't apart of this because they are marketed and sold as limited functionality devices. iPhones and iPads are sold as fully functioning devices (see "There is an App for That" and "Whats a PC?" marketing from Apple).

The USA has gone through this before with things like Bell Telephone and Standard Oil. They were too big and powerful that the only way to fix it was to break them up. The hope is that instead of breaking up Apple like those past examples is to prevent them from abusing their power and size before they get that large and dangerous.

Also, remember that Google is in the final processes of monopoly abuse lawsuits, Amazon is being sued for monopoly practices, Disney was sued for antitrust a few years ago, and Ticketmaster is being sued for monopoly charges. So, its not just only Apple here. And these are just the lawsuits from the US government, not even including EU nations, other companies, etc...