Gregor & Alex , your view is often voiced by many who have no clue what it's like to run a manufacturing business, or young uns who have grown up in the Information Age where everything is point, click and deliver tomorrow. I'm not picking on you.
Alex beat me to an answer with
No, it's not that it can't be done, it's that they can't be bothered because the margin's too low. And thanks for the "no clue" comment. You're always here to bring a shining light to everybody's day.
but some remarks anyway:
I'm fine with someone (or some company) telling me that they simply won't do something, even without giving a reason or when explicitely stating that they're just not interested as it would be an additional effort (however small) - even lazyness as a reason is acceptable, as it's an honest answer.
Simply don't want to do it for whatever reason? Fine.
It'll be more difficult to do custom stuff compared to off-the-shelf? Certainly.
The bespoke product might take longer to be delivered and possibly be more expensive? Reasonable.
Significant added complexity in carrying an additional dimension parameter in an order? Su... wait, what? In 2017?
I'm also fresh out of kind things to say to people which pull ad hominem, especially without having a clue about me as a person... Thanks
@Alex for wording it politely.