They didn't. I had a Honeywell tablet way back when, there is a company called hammerhead that makes tablets (http://www.yenra.com/rugged-tablet-pc/rugged-tablet-pc.jpg), panasonic has had the toughbook, etc. etc.
All of these are roughly the shape and size of an iPad. Apple just applied their standard solid-colored plastic design to it.
("GUYS! I have an idea! Instead of /white/ plastic lets...wait for this. Are you sitting down? Guys! Instead of WHITE plastic...what if we used....black plasitc?!?")
Even the iPhone. Palm had something that looked exactly like an iPhone in early 2000s. I remember my dad having one and giving it to me when it "broke". In fact, I think it only had one button on it.
At this point a tablet is as generic a shape as a laptop is. It's rectangular, about the size of a textook, and about a centimeter thick.
Apple: get over yourselves. If you're so /innovative/ then show us. Take more scrollbars off of things or something.