You won't get anywhere explaining it to them in technical terms. You need a metaphor. Tailor it to the person you're talking to, if you can. If he/she is a car freak, get them to think in terms of engine modifications. How much would it cost Ford to offer three different motors in the Taurus, or custom mods on demand?
Once they accept that comparison, even if they don't fully understand it, you can begin to get into why the metaphor applies.
There's another great way to help them see it your way- take some time to see it their way as well. When your paycheck depends on giving the customer what they want, you don't care what the propellerhead in Engineering tells you. If you're getting a lot of requests for customization, you should consider the architectural and strategic approaches to delivering those customizations, wherever possible.
