Do people not weigh themselves in stones and pounds?
Older people, usually yes.
Younger people, more often that not, not. Even at 48 I use Kg for my own weight, but those only a half a decade older more routinely use stone/measurements.
Though there is a sizable range of people who use one unit system by default but have a reasonable intuition of the other.
Unlike some things, there are no legal mandates dictating which set of measures to use for this.
Another difference in weight scales: we don't tend to work with just pounds when we use imperial measurements. When a US TV show gives a weight as, for example, “172 pounds”, many will need to do a little mental arithmetic (this may be subconsciously, not actively calculating but the process delaying understanding) to convert to X stone & Y pounds rather than naturally having an intuition of the weight from the single number.
It was always pounds and ounces when they were babies though. Not sure when it switched to kg; probably when we switched from "baby specialist" to "standard pediatrician" so around toddler age.
But it seems to change when people are old enough to be talking about their own weight.
Adult heights are the other exception, those are often in feet and inches. My 14 year old knows she's 5'2" but her knowledge of imperial measurements doesn't go much further than that.
I’m in New Zealand and we use imperial for baby weights, tyre pressure and height. Baking uses some measure like cups (US or imperial?) and teaspoons/tablespoons which I dislike, grams is preferable.
Surely the dumbest though is UK shoe sizing. The increments are barley corns length, a unit of measure which is hilarious. This is for males and children, women’s shoe sizing is apparently US. What a shambles.
I’m sure there are more niche hangovers. https://en.m.wikipedia.org/wiki/Barleycorn_(unit) https://en.m.wikipedia.org/wiki/Shoe_size
Wait, you can just use a unit-less value?
UK Govt Official Weight Form
Weight: well-nourished