Babies need more sleep than adults because they are smaller than them. The smaller the baby the more sleep it needs.
Young babies do not do much except sleep and eat. Babies grow while they are sleeping so they grow faster than older children. The Older children don’t sleep as much but need more sleep than an adult.
After a long day adults need rest to replace the parts of their body. While they sleep their bodies are getting repair to be ready for the next.
Adults need food for energy, and children need food for bones and muscles. The grown ups just need energy from food because of all the exercise they do. Babies just need bones and muscles to grow properly.