The "Create-your-own" concept

Growing up, I was taught that if I wanted something at a restaurant a certain way - that I should ask for it. If I wanted chicken instead of turkey, my fries well-done, or a lime with my iced tea instead of lemon (and in my dads case - "LIMZE-AH" - to emphasis the desire for multiple pieces of lime) that it was my right as a consumer to ask and receive that.
In some parts of the world, this is seen as a rude or pushy thing. Now, I agree it is all in HOW you ask, however, I firmly believe that a paying costumer should get what he/she wants, as long as it is in reason AND asked politely. Furthermore, if the requested change goes beyond what the menu offers, we should be willing to pay for it.
In New York and LA, I feel like most waiters expect to write a paragraph per customer on their pads rather than 3 words - but outside of these 2 cities, I feel like you are viewed as "that guy.
It is this belief that I propose to ALL every-day restaurants nation-wide (sans fancy places) to begin adopting the "create-your-own" concept. In New York, I have become hooked on the make-your-own salad. I give the servers a bowl of lettuce and pick-and-choose what vegetables, proteins, cheeses, dressings or whatever I want. The server then mixes it all up, puts it back in the bowl and charges me $8.00 MINIMALLY. But, this is fine with me - I'd much rather pay $9.00 for something customized exactly what I want, than $6.50 for the standard.
So, my question is why can't every place do this? Charge a base amount for the main ingredient (chicken, pasta, sandwich, omelet, sushi, whatever) and corresponding variable costs for all of the possible toppings or ingredients. If you wanted tomato, but not lettuce, you wouldn't have to pay $0.50 for "lettuce and tomato" you would just pay $0.25. It's truly a simple concept. I can't seem to think of financial reasons, inventory drawbacks, or anything else...so why don't places do this now? I have no clue. If someone knows why, please leave a comment. If you agree - then continue to ask your favorite restaurants for this. We are consumers and we are who decides which businesses succeed. If we want it - they will offer it!