> Why didn't they also add the an arbitrary precision type as well?
My take on this is a) you're not thinking like a business, and b) product teams don't have infinite resources, even at Microsoft--there are probably better things for them to spend their time on.
I have seen Eric Lippert respond a number of times to the question "Why wasn't feature X implemented?" A couple of his public posts that touch on the topic: How many Microsoft employees does it take to change a lightbulb? and In Foof We Trust: A Dialogue.
(Eric's blog covers many topics, and has some good series on immutability and covariance/contravariance. One could do a lot worse than to read his blog regularly.)