Earlier quoted context omitted.
I assume those people either fly to and back in the same day, have an already established "base" with clothes & cosmetics on the other end, or just buy everything at their destination. Personally, even for business trips longer than 2-3 days I end up having to use checked luggage.
I'm leaving today for a 4-day trip with just a backpack (I'm a guy without contact lenses or other medical issues so I need minimal cosmetics) (admittedly I'm taking a bit of a risk by trusting the weather forecast). A few years ago I went for 10-day trip to Cuba (i.e. you can't really buy stuff there) with a 40L backpack.
Nice. But how did that fit in the usual size & weight limits of hand luggage? Or did you fly with a decent airline instead of a cheap one?