1. The ancients pursued grand connotations in naming and did not like to use uncommon characters, such as Guan Yu and Zhang Fei; modern people have low literacy, so they cobbled together uncommon characters such as Xi, Xin, and Ning.
2. Ancient Chinese is inherently complex. What we consider rare characters now were often not uncommon characters in ancient times. It’s just that times have changed and these characters are no longer used.