I used to work for a hair-care company. In the past (and this is 15-20 years ago or more), you could find shampoos for oily hair – there was even a brand, FOHO, “For Oily Hair Only”, and many other brands had a variety for oily hair.
The market for such products was always small, but it seems to have decreased even further, to the point where (as you’ve discovered), it’s very hard, if not impossible, to find “oily hair” shampoos any more. There are likely a couple of reasons for this:
People wash their hair more frequently now. 30 years ago, it wasn’t common for people to wash their hair more than a few times a week; today, there are many people who wash their hair daily. Shampoo tends to dry out hair, someone who feels that they have oily hair if they wash it every other day may not feel that it’s that oily if they wash it every day.
Heat appliances (such as blow dryers) dry out hair, and are very commonly (and frequently) used now.
I’d suggest looking for a “clarifying” shampoo; they’re meant to clean build-up from styling products (and thus, have a high detergent level).
BTW, just another observation: bar soap and shampoo are two different sorts of products. Shampoos are actually detergents (closer to dishwashing detergent than anything else).