Why is it natural for older people to withdraw from society?

There are several different reasons they might do so. They may not be working and shaping the future, so they may be less interested, especially since they know they probably won't be around for as long as younger people. The future may, thus, interest them less. They may be more comfortable with the version of the world they and their peers were more powerful in shaping.

Since many will be retired, it's natural for them to focus on personal interests instead after many years of dealing with the public. Many will take time to reflect on their experiences. They may have ways of doing things now considered "outdated" or old-fashioned."

Physically, they may be ill or simply have less energy.