I think you severely underestimate the cost of hosting servers for ~46.000 Games, across 9 regions, in 190 countries, with 500Mbit download speeds. On top of that billions of screenshots, trillions of lines of text, customer service, development of new features and hardware, etc.
Valve has an est. revenue (not profit) of roughly $10 Billion this year. Tencent Games has an est. $85. How is Steam even remotely considered to be a monopoly in gaming?