I can tell you why... which should be a no brainer... all of USA uses imperial units. An american made product that does not recognize imperial units is strange. My question to you is why doesn't it accept imperial units?
To have the option to choose which units you would like to draw in should be standard in all construction programs.
Every day more and more items are leading to me find this program to be a complete waste of money.
You have asked this before. You must be located outside the USA. We have only used Imperial for the 30+ years I have worked as a structural designer. I was very happy to see a replacement for Bentley Structural and amazed that SD was lacking Imperial units in just the formwork module. Can you explain why you don't see the need ? I had hoped Autodesk would be working on this but your question indicates otherwise.