Collusion often results in this, which is not a dejour monopoly, but a defacto monopoly. Collusion isn't even strictly necessary. Endstage capitalism naturally selects the winners who stay on top. It's a simple concept taught in beginning economics.
"big companies" == oligopoly == monopoly is exactly what people mean when talking about worrisome "big companies", without the commensurate analogies.
A monopoly is about perspective which is about scale not some subjective definition (like sanderjd endlessly trolls about). Legal systems all over the world combat real/threatened monopolies (eg the CCP recently acted unilaterally) in various forms. Whether you think it's a useful reasoning to group companies and use a specific term is irrelevant. I sincerely hope you can understand that.
Good luck with whatever.