So say you play a tournament and have to play Rivers. Now since you dont know how the map will look like, you cant prepare a strategy. What civ do you pick? How reasonably fast can you even check for surrounding water? Gotta find sheep/boars first etc.. Theres a reason Megarandom ( i assume thats MRm) is not a competetive Map. It can be fun to play i agree, but has no place in a competetive mappool. Same goes for maps that can have different Layouts.
You can prepare a strategy by identifying the ideal way to scout on a map with an unknown layout. Similarly, you could pick the civ that works best across all options. These are also skills.
And the comparison with mega random doesn’t work, as this is really random whereas the few different layouts of rivers are easily learned.