The amount of CO2 in the atmosphere is around 350 ppm or so. If still water is allowed to fully equilibrate, you'll get a concentration of 2-7 ppm (depending on a number of factors, especially temp). Something else must be at work here.
I believe the general consensus is that the majority of CO2 comes from two sources: groundwater and decomposition. Ground water will carry in CO2 from topsoil and from reactions between acidic water and carbonate based rocks, such as limestone. The biggest contributor is probably decomposition of organic matter on the bottom.
I found this nifty little chart on the University of Wisconson - Madison website:
Time of Day, ppm
6 am, 23 ppm
9 am, 16 ppm
12 pm, 9 ppm
3 pm, 6 ppm
6 pm, 3 ppm
So CO2 levels do get up to around recommended levels and are slowly depleted during the day. It's also interesting to note that while the overall levels are somewhat lower, the sheer mass of water means that while their may only be 23 ppm available at any one spot, there is a huge amount of CO2 available to the plants.
I may also be worth noting that natural ecosystems have no sense of aesthetics. Algae can run rampant, which is something that we generally strive to eliminate in our tanks, further backing the need for supplemented CO2.