Why does mexico not have a river system that crosses the country?

Mexico has two mountain ranges (Sierra Madra Oriental and Sierra Madre Occidental) that run generally north-south along the majority of the mainland (that is, everything other than the Baja Peninsula). If you look at an east-to-west cross section of Mexico's landforms, you have the coast, then a mountain range, then a BIG valley/basin, then another mountain range, and finally the other coast. 

Answerequatorial guinea,gabon, and western sahara;