Line drying was not common in the 1990s (in the US) when the claim is underwear lasted. Washing machines have switch to low water use versions, so that might be a factor (I doubt it, but it the only other thing I can think of)

Line-drying is very different from "hand-washing".

I "line-dry" (drying rack) a lot of my clothes (pretty much all my dress pants, dress shirts, anything that I don't want shrinking in a hot dryer). But I don't use a bucket or tub to handwash and handwring my clothes; I still machine-wash on delicate with cold water even if I line dry. I've never met anyone who hand washes, in any country (though I know it's popular in many less-developed nations, I haven't met anyone who does).

I have occasionally handwashed very expensive wool sweaters, but I prefer dry-cleaning them, not only because of the labor but also because if I let them soak even just a little too long the wool fibers start separating and permanently changes the sweater.