I've noticed that for certain recipe kits that come in both all grain or extract forms, that the directions for the length of fermentation (both primary and secondary) are generally longer for the extract kits. Why is this exactly?
There is no particular reason I am aware of that normal fermentations for extract vs. all-grain brews should be different. Perhaps some examples that you've noticed might help?
Fermentation time is mostly a function of yeast health, wort oxygenation levels, yeast pitch rate, the gravity of the beer, and temperature. While extract might generally have less essential nutrients for yeast (I'm not sure, honestly, but I could believe that to be true), the other factors make more of a difference. Notice that almost all of those are nothing to do with the source of of the wort's sugars.
(Also, I wish the major retailers would stop recommending "secondary" fermentation on their instruction sheets. It is almost never needed.)