It's not just Americans on the web. The whole point of the metric system is to have a standard that everyone can understand. If you have trouble with it, just do a quick google search for a converter.

As far as why it's done. I imagine it's the same reason, as a lot of breeders ship overseas. Even in the USA the metric system is used in medicine, science & technology, and it is starting to make is way into construction and architecture. In my opinion, the USA should get with it and join the rest of the world and adopt the standard.