In the off chance of damaging your ego, I can say I've never heard of you. Therefore I feel I can give you an unbiased answer to your question. Now I've only been studying Uechi for about six years, and there are many people who frequent this web site with much more experience in Uechi then myself. So If my explanation isn't very clear then I'm sure someone else will jump in to help out.
Let me start out by saying that this is my interpretation of Sanchin with out getting too philosophical. It might not be everybody's, in fact it might not be anybody's.
Sanchin is not a Kata like any other. Your right when you said it seems like a simple Kata. On the out side Sanchin looks like a very simple Kata. You throw a strike you take a step, you throw a strike you take another step, it doesn't get any more simple then that. But the longer you study you will find it is the most difficult Kata to "master". When performing Sanchin you are not imagining defending yourself against invisible attackers like you might do in other Kata. In a physical sense Sanchin is where you work on all of the building blocks of what makes Uechi-ryu, Uechi-ryu. It is where you work on your stance, your "center", your ability to make a powerful strike look completely effortless. Even the ability to take a strong strike to the legs, arms or stomach without losing any of the criteria of what makes a good Sanchin. I'm not sure if you have ever witnessed someone having there Sanchin tested, but it can be very impressive.
On a mental note Sanchin is used as a meditation. Working on a strong "Zanchin" (sp?) your focus or composure. It's used to force yourself to concentrate on a particular moment, it's reason for it's "simplicity" as you put it. You might have herd in your months of study the all is in Sanchin. Well it is. Everything you work on in Uechi is related to Sanchin. The way you step, the way you throw a hook punch even the way you kick, all can be seen in Sanchin. If you are getting comments about a certain problem in one of your Kata, the same problem can usually be seen in your Sanchin. Some people like to think of Sanchin as a teaching tool, but I like to think of Sanchin as Uechi-ryu. When you work on Sanchin you are working on all of your Kata simultaneously.
This may not explain Sanchin very well, but I hope it may intrigue you too look at Sanchin a little deeper the next time you are performing it.