"Oh this is really frontier space, you can smell your food cooking!"
It's illogical to boil a pot of liquid by shooting flames up the side. I think Neelix did it for the theater. He wants people to notice all this dramatic preparing of foods. Most of the time you don't see your food cooking, the kitchen of a starship or work cafeteria is safely tucked away for hygiene and safety purposes. But Neelix doesn't want people just retrieving sustenance like from replicators he wants them bedazzled by him providing for them so everything is visible and everything is fiery and exciting.
And I bet most of them are so far removed from non-replicator food prep that they haven't a clue that flames shooting up the side of boiling pots makes no sense at all.