I think a more appropriate answer would be - because things are better with gravy all over.
You are probably reffering to the delicious Japanese curry sauce which is their main feature and draw. Their curry sauce can be ordered to the side but it's best how they've intended it.
It's a curry house. They serve Japanese curry. What were you even expecting? Of course it has curry all over it.
Japanese Curry looks like gravy but is not. You can try requesting them to put curry sauce on the side or to put less of it.
It's not gravy, it's curry.
It's curry, and you have the option exclude the curry with your meal.
You can request it on the side
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