Can someone explain the shoe size mystery to me?

I went looking for some old tennis shoes at Goodwill for wading in the lake, for my dad’s wife, who is coming to visit. She said she wears a size 10. Why isn’t a size 10 in women’s the same as a size 10 in men’s?

I found some shoes that I think will work. They like what yacht captains wear. They just slip on. I can’t tell if they’re men’s or women’s, but they have “Size 9.5” on the tag inside.
I measured the soles with a ruler, and they’re 11.5 inches long.

Why don’t we just use regular inches for ALL shoes, from kids shoes on up? Can’t they just measure the damn things and call it what it is?