The CBP officers are trying to identify drug smugglers and other who would do harm to America. The most efficient way to do that is to initially screen people entering the country by asking questions about why people are traveling and who paid for it. If a young American student re-entering the country says "a Peruvian guy I met in a nightclub paid for me to travel alone to South America for no real reason at all" then they're going to search that person's bags. You can't expect them to search everyone's bags, nor should you expect them to only use random bag searches. That's not efficient. Screening by asking questions and then searching the bags of people who seem nervous and who have stories that make no sense works. It helps keep the country safe.