Here's why:
* Indefinite pronouns: Words like "everybody," "anyone," "no one," "someone," "everything" refer to an unspecified person or thing.
* Singular treatment: Even though they might seem to suggest a group, indefinite pronouns are treated as singular in standard English grammar.
Examples:
* Everybody is going to the party. (NOT "Everybody are")
* Someone has left their phone on the table. (NOT "Someone have")
Important Note: Some people might use the plural form in informal speech, but in formal writing and speaking, "everybody" always takes the singular.