Why doesn't Riot use AWS for game server instances?

Riot said that they already tries to find players with similar ping in matchmaking, so players are indeed matched by geographic locations in any case.

Within the same region, Overwatch uses AWS servers in tandem with Blizzard's own servers. Why can't Riot do the same and distribute server instances in different geographic locations, and just do some form of load-balancing according to different ping?

I mean, this seems to be the obvious thing to do for any right-minded developer of a PvP online game, why did Riot decide to stick to a single centralized server for all game instances in the first place?