First of all you should ask why do subject and verb agree in number.
The subject in your question is pronouns; it is plural. The verb is to do and in the third person plural the form of the verb is do. For this reason your question should read, "Why do personal pronouns....".
All nouns and pronouns have number. If your question above referred to pronoun, it would be singular and would require a singular verb such as does; since your question refers to pronouns, which is plural, it requires a plural verb.
Personal pronouns also have gender: he is masculine, she is feminine and it is neuter. Personal pronouns should agree in gender. It would be incorrect to say "Alice took his bath on Saturday night" unless Alice was a man.
Personal pronouns also have case. He went to the store, NOT him went to the store. He is nominative whereas him is subjective. Worse yet: "Him and I went shopping. The correct sentence would be: He and I went to the store. For the subjective case: Brittany gave he a gift. No, Brittany gave him a gift. In this sentence the word him is called an indirect object whereas gift is the direct object.
OK, you asked for it. You got a short English lesson for your trouble. Now why did I bother to answer your question? If you don't remember the specifics of my answer above, please remember this. The English language is a beautiful precision tool which.allows you to express exactly what you wish to communicate. You should learn to use this tool well and not abuse it as your question did!