Well, yes I have noticed this too but what´s the matter?
I started out recently in the middle of a tiny island
covered with tundra.
Isolated and with only 4 cities I thought I was doomed...
however, once I met the others (who at that point was way
into the middleage-era while me still struggeling with the key
technologies of the ancient era) I was able to come back...
I had a lot of money (as anyone has starting the game isolated)
and was able to buy myself back into the tech-race.
I still was Isolated having my 4 cities and envied those egyptians having rivers criss-crossing their massive, fertile grassland continent. But egyptians are punished for this - they pay up to 30 times more for their techs than I do.
I am not able to grow my cities to as a high size than the egyptians are but why should I? There seem to be no need to
do so. My level 3 coastal city (unable to grow any futher) had already finished 2 wonders of the world and is ranked first in the top 5 cities - yes, a level 3 city
(Low growth = high prod)
My point is that those rivers, while causing income, also rises
tech-costs for this same reason becouse the CIV beeing at the river is more likely to lead the tech-race - so practicaly speaking they are useless. Everything that boosts a player (or A.I) is more or less useless since there is a mechanism in the game that counters it. Thus having only those 4 cities on the tundra
or 20 cities on a massive, fertile continent matters little.
Actually, In my game it does not seem to matter at all...
-Saurus