Why am means morning and pm means afternoon?

Because am and pm are Latin abbreviations for morning and afternoon.

morning (A.M., Latin ante meridiem means before noon) and afternoon (P.M., Latin post meridiem means after noon)

Extended materials

AM and PM can be used in both English and Latin, and often appear behind digital time. There are two formats for expressing time by numbers in English. For example, 8: 3 can be either 8: 3 or 8: 3 in English. The former is universal, and the latter is a traditional British representation.

reference:

Baidu encyclopedia-am

Baidu encyclopedia-pm.