A lot of it stems from the fact that different brands and types of contact lenses - even ones with the same name - vary widely in terms of material, oxygen content/permeability, and actual physical dimensions of the lens (the two variables are called the "base curve" and "diameter"). The diopter is just one axis out of 5 or 6.
A given lens in one patient will not always be comfortable or even stay adhered to another one's eyeball. This is why doctors do "fittings" where a trial version is placed and observed for a few minutes or a few days to look for side effects. Lenses actually need to move when you blink, the doctor looks for that as well. Lenses that don't move can deprive your cornea of moisture and oxygen and scar it.
That said, once you're on a lens the matter of having it renewed is a simple function of your annual examination, which is something you should be doing when using contact lenses. If you're one of those people that make 2 week lenses last 6 months at a time and sleep in them, or you are buying them in gas stations, you're risking a lot. I sleep better knowing mine are FDA regulated and recalled if problems or contamination occurs.
I'm not a ophthalmologist, but I've been wearing contacts for nearly 30 years and I respect the process. I'd rather wear 1-days for the rest of my life than undergo LASIK.
