Why shouldn't we simply keep our technology secret?

In addition to the answer provided by Mike, if somehow your designs and code became known, someone else could apply for a patent - the US went to a first-to-file system in March, 2013, so unless you can also prove that your designs and code were in place/in public/commercial use prior to this other person filing for patent, whereby your use would effectively be prior art to his patent application, they may receive a patent and then sue you for infringement. In addition, even without someone else getting a patent, if, again, your designs and codes were known to others, they could simply start using them without fear of infringing your patent because you would not have a patent.