They do it for the looks. They think others don’t care about legs and they don’t like training legs so they don’t. Truth is exactly how you said it, it looks extremely unbalanced and most women/men I know actually prefer people with a nice lower body.
The flip side are women who only train lower body and it’s the exact same thing. They look lopsided.
As for you mentioning using strength to lift irl stuff, very few people these days lift for functional strength. It’s for health and looking good 99% of the time now. Usually that’s mentioned by people who don’t really work out which is interesting now that I think about it.