Why is there so much oil in the base gravy?
It's a very good question.
In my opinion it's because oil is the primary flavour carrier for spice essential oils to dissolve into. Not much point putting spices into a base sauce if there's no carrier for them or anything for them to dissolve into.
This is why (in my opinion) BIR style curry nearly always uses a lot of oil and why the resultant takeaway dishes are often swimming in it.
BIR style cookery and dishes aren't exactly the the most healthy out there. But it's because of all that oil the dishes taste so yummy!