Why does English grammar require 'has' with third person singular subjects?

English grammar requires «has» with third person singular subjects, because the verb needs to agree with the subject in number. linguix.com If the subject is singular, the verb must also be singular. linguix.com

Third person singular subjects are pronouns like «he», «she», or «it» that describe one single person or thing. worldenglishblog.com www.englishteacherkay.net These subjects are followed by verbs with an «s» or «es» added to the end. worldenglishblog.com www.englishteacherkay.net

In the case of «has», if it is preceded by an auxiliary verb («does»), it loses its conjugation, but when there is no auxiliary, it is conjugated for third person singular. ell.stackexchange.com

Example: «She plays the piano» — «She has played the piano». worldenglishblog.com linguix.com