In a market where someone is making billions in profit, it's almost de-facto evidence of a lack of competition. Not a priori, obviously, but it gives you a basis for measurement that where you're looking is somewhere on that spectrum.
For big box stores, that razor thin margin is exactly what you would expect from a very competitive market and so it's fair to say that it is one.
Who else can charge 30% on every sale just because?
Every physical product retailer charges > 30%. This is what I'm suggesting is the confusion point. If an app developer sells an app for $0.99, then the dev gets $0.66 per transaction. If a physical product producer sells something with an MSRP of $0.99, the retailer will be purchasing the item at a wholesale price of closer to 50% of the MSRP (if not a higher percentage) leaving at max $0.50 per item. This shows how much easier the software dev has it.
There are costs of doing business. They can all be negotiated or at least attempted, but at the end of they day you want your product where the consumers are, so you have to ultimately play within their terms. You can make 70% or make 0%.