AWS gives you mainly a blank slate. You could try pushing game data through services such as SQS or Lambda, but those services really are not intended for game-style use cases -- they work best for bigger, less frequent, less real-time sensitive events.

In general, the N-squared problem of "every player could interact with every other player in a town square" is the biggest technical challenge in an MMO. Solving that requires a careful attention to game design together with technology. Many games have died because they don't get it right -- either the design is not really technically achievable, or the tech drives design to the point where the game is generic and boring.

Thus, most MMOs can make a good argument for needing at least partially custom network code, because it's so core to the final experience.