Because charts take alot of time to make, and are generally not as popular as reviews because they are just results.
The charts are there just for a general Guide line, you look at the card that does the best and then you go read a review about it and see if the numbers are justified.
Check out the charts are Guru3d.com
Tom's Charts should be updated soon, its been more than 6 for sure from what I know
they atleast update it once a year I think I can say that much