Because it's probably not at all cost effective. You're talking about taking floor space and machine time away from producing standard models in order to fulfill custom orders. It's not just a matter of assembly. Extra parts have to be made too.
Time is a very important factor. Normally you'll probably have a production schedule to meet a quota. X machine makes N number of part Y, which is then split between a specific set of different models in predetermined numbers. That same machine is likely then reconfigured to make a predetermined number of a different part. Everything is budgeted out this way to track and control costs. If you want to do custom orders, it's going to disrupt all of that because it's introducing an element of randomness into the mix in the form of an inconsistent and unpredictable demand for parts going towards custom SAKs.
Either you're going to have to create some new system that allows everything to be reorganized on the fly somehow, or else create a new facility dedicated to producing custom orders, which introduces a whole new set of costs and other difficulties.
Those are just my thoughts anyway.