it's not 100% monopoly; there's some alternatives that hold a tiny share

You're just describing a cartel with a monopoly.

And probably it's not exactly a cartel since they don't have hidden meetings to discuss the prices.

Why on earth would you believe that? Do you think email, IMs and phone calls make it less likely now? The Phoebus cartel had secret meetings in the 1920s and they weren't making money like Bootleggers or Banks. Do you really believe they were an isolated case? What do you think Enron was? What do you think industry conferences are for? Real estate, oil and gas, airline and auto industries, and so on, are all famous for manipulation. Microsoft has for years made computer makers pay them license fees even when they don't use their software. LG got slapped on the wrist for keeping lcd screen prices artificially inflated, and a bunch of Korean companies (Hynix, Infineon, Micron Technology, Samsung, and Elpida) companies got caught fixing DRAM prices, and you can be sure bigger, richer more sophisticated western companies like ATT, Comcast, Dell, IBM and Apple wouldn't get beaten at that game by bit players like them. People have been complaining about merchants conspiring in public since Adam Smith and I'd bet he had precursors (maybe the prices of barely or wheat were fixed in Mesopatamia).

People of the same trade seldom meet together, even for merriment and diversion, but the conversation ends in a conspiracy against the public, or in some contrivance to raise prices