Because they do not have anything else to say, they are hungry, or they just are cool enough to say that in general.
The Cookie Monster when he eats say "Om nom nom" and people started using the phrase
No
om nom nom
You chomp it all upp om nom nom nom nom
they make your taste buds say OM NOM NOM.
Om nom nom
om nom nom nom
Me! Om nom nom nom nom!
James hart is ginger omnomnom AS MANY FECKING OM NOM NOMS AS JEBUS WILL ALLOW IN A SINGLE CONTEXT OF THE HEBREW JEWISH DICTIONARY OF SHETY COW PATS ON TOAST !
om nom nom
Pie will be eaten... by me! OM NOM NOM :P
Three words: Om nom nom