Because it was agreed upon as what would compose an international standard. There are exceptions, however to everything, such as bike tire/wheel sizes, car wheels/tires, ratchet drives, and a few other things, as the measurement method became a somewhat even number in inches that does not easily correspond to mm or cm.
However, it looks like you don't know this, but metric is taught in US universities and colleges, with Chemistry, Physics, and related courses, as well as more specific courses above being taught in metric only. They sometimes use non-metric units in order to add complexity to a problem. However, the rest of the problem is conducted in metric.
So the US is a metric country, with some crossover, however. Some things will likely be leftovers, such as miles on roads, but moving over to metric is a good thing. Hell, I have never owned SAE tools and never needed them.
