I think what you're really asking is: Where's the value if I want to (or have to) do my own marketing anyway? The answer is trust. I'm more likely to trust you with my money, with you executing code on my device, if Apple allows you on their store. Even the big brands like Epic get this value, because who is judge that whatever "I look for" isn't being hijacked by some impersonator? Perhaps that trust is misplaced, but the effect is real, thus the value is there.

Apple considers this worth a 30% commission, and 100% of developers on the App store agree, or they wouldn't be on there. The question whether you couldn't get more value by doing it all yourself doesn't even ask itself, because there is no alternative. Whether that is anti-competitive in the legally prosecutable way is a very interesting question that we're all eager to hear answered by the courts.

Personally, I do find that a 30% (or 27%) commission on out-of-band purchases is outrageous and that phone platforms should be cracked open, but I can also see the argument from the other side, comparing e.g. to video game consoles or set top boxes.