TTB, one reason is the additional cost for hold baggage on some airlines so it is both cheaper and more convenient to have hand lugage only.
Yes you can buy what you need in a shop; shampoo, shower gel and shaving cream, but Mrs TTB, and certainly Mrs PN, will not want to take the risk that they cannot find the right colour nail varnish, hair spray, shampoo, deoderant, perfume, nail polish remover, toothpaste, lipstick, lipsalve, sunscreen, insect repellent, hand gel etc etc.