Why is muscle force more dependent on cross-sectional area than on fiber length?
Here is a thought experiment that might help. Consider a single muscle fiber as being like a rope. Granted, a rope won't shorten on its own, but if you think of an isometric muscle contraction, then it's closer to what is happening when a single fiber contracts. 

Now think about two situations:


You double the length of the rope. Does the total force that the rope can hold get larger, smaller, or stay basically the same?
You halve the length of the rope by doubling it up (so two ropes instead of one). Does the force that the rope can hold get larger, smaller, or stay basically the same.


So lining up more fibers in series is like making a longer and longer rope. It's still basically as strong (or weak) as a single rope. But when you bundle more ropes, increasing the cross-sectional area, the overall force that they can hold increases.

I'm not sure why a thought experiment needs citations, but here you go:


The canonical source for discussion of muscle fiber architecture is Gans C. 1982. Fiber architecture and muscle function. Exerc Sport Sci Rev 10:160–207.