Because 7 said to the next number “Yer an 8”.
Explanation: This joke plays on the pun between “you’re an 8” and “urine 8,” suggesting that when 7 complimented the next number (8) by calling it attractive (“you’re an 8”), it sounded like “urine 8,” leading to the humorous idea that 7 was covered in pee. The wordplay on “urine” and “you’re an” makes the joke funny through its clever sound substitution.