Here's why:
* "Somebody" is an indefinite pronoun: It refers to an unknown person. Indefinite pronouns typically don't have possessive forms.
* Possessive forms show ownership: To use a possessive, you need a specific noun that can own something.
Example:
* Incorrect: "Somebody's book is on the table."
* Correct: "Someone's book is on the table." (Here, "someone" is a more specific indefinite pronoun.)
If you want to express possession with "somebody," you need to rephrase:
* "The book of somebody is on the table." (This is awkward and not recommended.)
* "Someone's book is on the table." (This is the most natural and common way to express the idea.)