the simple asnswer is because people like you can't wrap there little brain around the metric system with such complicated things like the freeing point being 0 and boiling at 100 good thing your sitting down huh?

the long answer is this

In the 1970's there was a major effort to increase the use of the metric system, and Congress passed the Metric Conversion Act of 1975 to speed this process along. However, American consumers generally rejected the use of metric units for highway distances, weather reports, and other common measurements, so little was accomplished except for the encouragement of faster metric conversion in various scientific and technical fields.

here is the link to the page
Units: The Metric System in the U. S.