I would like to thank @Rick Christopherson
for his excellent points, which I had not fully considered. The issues are not only about common units of measurement. (For "standard" woodworking, I use inches, not feet and
inches.) Rick also pointed out that when we do woodworking in standard units, we tend to use fractional notation, but when we work in metric, we use decimal notation. I am comfortable with either notation style and the math that goes with them, but I can see how switching between them could be more challenging for others.
Since I got back into woodworking a few years ago, I tell my spouse how surprised I am at how much math I have needed - geometry, geometric construction, and trigonometry in particular. I store a calculator with trig functions and a small drafting set in the garage, and use them. A compass, dividers, and straightedge can do a lot. I like the old vintage US and German drafting tools, and bought a small vintage drafting set on eBay for around $10 to keep in the garage. I suspect that most woodworkers don't bother with math and drafting tools. A practiced eye and a lifetime of experience is faster and better.
I would also like to thank @NoThankyou
for giving a plausible reason for why we didn't make the expected switch to the metric system in the late 1960s and early 1970s. It makes sense to me. I like to cook, and I bake the bread for the family, so I get it. I still use teaspoons, tablespoons, and cups in fractional notation. I would be happy to switch to metric, but all of my recipes (and those I find online) are not. For the last two decades, many volume measurements are done by weight, using a digital kitchen scale. It faster and far more consistent.
For those who grew up with metric cooking, 3 teaspoons = 1 tablespoon, but 2 tablespoons = 1/8 cup. Ouch.
In response to @JayArr
's post, I have never heard of anyone using radians or grads for angle measurements in woodworking. Most woodworkers have never heard of either scale. I have seen posts here mentioning "dms" notation, but my work has never needed precision finer than 0.5 degrees. The need for 0.5 degree precision happened only once so far, but the 0.5 degrees made a noticeable difference for that project.
I still wish we could bite the bullet, pick a day, and switch to common international standards here in the US. It will be a painful and expensive investment, but the obvious payback should appear in less than a decade, two at most.