Why do counties end in shire?

Answer 1

The word "county" only came into use after the Norman invasion of 1066. Before that time "shire" was used, they mean the same thing.

The "shire reeve" was the local nobleman's officer in charge of keeping the laws enforced, this was corrupted into "sheriff".

Answer 2

Counties that end in "shire" typically have roots in Old English language, where "scir" meant a division of land or administrative region. The term has historical ties to governance and administration dating back to Anglo-Saxon England.