Simply because there are two systems – a U.S. and a metric ratchet set — are both in use right now, and there is little chance that there will be any standardization in the next generation. It is true the U.S. is committed to the metric system, having made that clear in legislation during the 1980s, but passing legislation mandating metric systems and getting mechanics that have been schooled in inches and pounds-feet are two different issues. Getting mechanics to use only a metric ratchet set isn’t likely. It seems that there are as many people fighting the metric system at times – because it wasn’t invented here or so they think – as there are those who use it. Just coincidentally, the U.S. currency system is primarily metric.
For more info, see Auto Ratchet Set: US or Metric? Which One Should You Buy