> Why are we so arrogant as to think that our design is the only design?

We're not. Nobody is. The reason scientists don't look for life on other planets that is made of other chemicals is that we may not be able to recognize that as life. It's much better - read, scientific - to look for something we can recognize.

Also, there are chemical properties of the elements that make up life on Earth that allow for fast reactions, bonding, etc. Most of the rest of the elements in the periodic table react more slowly in normal conditions, making the process for life less likely. Not impossible. Just less likely. We think.

So we focus on looking for what we know about. That's the best way to do it. Nobody worth their degree will ever claim that our "design" (bad word choice, by the way) is the only one.