Presumably because the regulations were written by people who have zero experience of either kitchens, fires, or kitchen fires.
Do you know what happens when you put water on an oil fire, which is pretty much what you're going to have in a kitchen?
You seem to erroneously believe these are a little squirt gun pointed right at the oil pot.
> Do you know what happens when you put water on an oil fire
When the water is coming out of the ceiling at 25 gpm, you get a lot of slightly oily water and no fire.
To answer your earlier question, the purpose of sprinklers in a house is to save my pets' lives if a fire starts while I'm away.