Why is service charge pegged to size of total bill?

Just struck by this question and realise I actually have no idea but blindly paying 10% for the past N years.

Do I get more service for prawn fried rice instead of pork chop fried rice? Don't think so right... but because prawn fried rice more expensive, I'll have to fork out more service charge in the end.. Might as well just hike all the prices by 10% lar.

If say because overseas give tips based on 10-20% of total bill... that's their business really. They blind doesn't mean we also have to be blindly paying service charge.

Maybe I just didnt grow up with the concept of service charge and needs some justification for this. anyway lots of services at restaurants sucks.