Because it's not "हम केले और एक सेब खाते हैं " Context. I feel if it's we eating just one apple, the number would be more emphasized. Since it is we, one would more likely assume more than one apple.
You are wrong, In last there is '.' on है = हैं, हैं means the sentence is in plural form (there is no plural word to सेब) I hope u understand. My whatsapp number (+917989715056)we can clear doubts
Read again. I did not say it was only one apple. I said it is plural. Reading comprehension is important.
Further, the plural form of hona is used, because it is connected with ham (we). It has nothing to do with the word apples.
I think "we eat apples and bananas" should be right as well. It does not change the meaning of the sentence.