Because the costs are way out of proportion to accomodate that single piece of #%@$%#, besides how would you like it when almost finished with your work that some boss's interpretation of your work is way of bounce causing you to have to do the thing again for a large part within even less time. It will make you hostile, just like most of us web programmers are towards IE.
Oh yes, IE has nothing to do with personal preferences, they refuse to stick with international standards and till recent get away with that behaviour, they will not in the future and you will hear less bitching from us when that future becomes today.
I understand your point but don't share it. What I have seen is that designers tend to ignore IE in their tests until the end. It is not like IE appeared suddenly and then you have to redo your work.
I believe those tests should be done incrementally in multiple platforms to make sure the design works in several browsers from the beginning, not at the end.
Why? Because one player does not want to play along? If I design something in Firefox then I am able to test in most other browsers like Opera, Safari, Chromium (Chrome) at nearly the end of my production and the correction would be in a margin of 0,01%, why would I test every single step and throw away my valuable time and money? And why should I keep my mouth shut? because you tell me to? Don't let me laugh...
Next time don't ask the question if you don't like the answers...