Why do people in some cultures not care about what they wear?

Depends on the culture. The South Sea Islanders had no real reason to wear clothes. The temperature was a constant 80 degrees. They also had very little clothing material available before the coming of the White Man. The same thing goes for the natives in the Amazon Jungle and the Caribbean. It was the coming of Christianity thar put an end to nudity in Europe. Before that the Olympics were run naked. Taking a dip in the Aegean without any clothes on was relatively common.