Becasue I ran the ones on riftval to fit my base placements that I wanted bridges between the spawn and field. Along with running at the bottom of the valleys I accommodated for the restrictions of the base placements created by Hitech's rules for MA terrain base placement. On Oceania which was before Hitech introduced the road\river\train paths objects, I hand cut rivers in to accommodate using bridges as GV choke points for attacking fields with the map room on the field. Hand cut rivers narrow enough to be spanned by a bridge object, run into a problem of the cut path being limited by the direction you are cutting across the ground's polygon field. On oceania I suspect you have seen those tiny polygon teeth that really cannot be smoothed out when the river is narrow enough to span with a bridge object.
Laying down river paths with the path object that you see on riftval becomes limited like getting flat roads by the polygon field in 3D spaces in a descending valley bottom between two ridges. And by the needs of the horizontal topography profile to achieve a design visual. The terrain mesh for AH3 is made of 660ftx660ftx660ft polygons. The vertices meets are unforgiving if you want to retain an illusion of smoothness on a valley bottom for a river path or a road path. After all you don't want your river object or road object to have vertical vertices kinks in them. Riftval was the first time for anyone using the new path tool for a large scale terrain and by fjordma I chose not to bother becasue of the amount of time a single river takes including bridges.
Anything else you want to know about terrain building for the MA? So did you really want to know any of this or just scold Hitech for something he didn't do?
Hitech does not build the terrains for this game, players do. Aside from the four I created for AH3 and buzzsaw that I think Waffle created. The players who created all the old terrains in rotation are not in the game anymore except for Greebo and his CraterMA. And mine are the only ones with intractable rivers visa bridges on them. CraterMA and Grinder have very wide rivers more like brackish bay meanderings because of the widths.