So..? What's your point? These developers are obviously happy to let their products, their own promotion or word of mouth speak for themselves and don't need Apple's "advertising" that's been forced upon them.
Wrong.
They're also providing the "storefront and till" so to speak, as well the tax processing where required.
2Checkout "as an authorized reseller" - so as the seller - for the software company I bought from.
The ones that Apple is giving out for free anyway, you mean?
As I said, some developers or companies are perfectly fine without Apple's hand-holding and supplementary services being forced upon them.
I don't believe I am wrong. Lets take your example, 2CheckOut. When I check their website they don't offer a store front, they interface with existing backend store solutions like shopify (they support 120+ carts which is quite impressive). They do, of course, offer a 'till', this is what I mean about payment processing, but you still need a store front such as Thrive, Shopify, etc. Their payment processor will generate links, so the shop front doesn't have to be terribly complex or expensive, you could do it using a free version of WordPress. Their pricing, though, is not based on a fixed percentage, rather there is a minimum charge and a percentage.
If I compare 2CheckOut with Apple, then I would need to use their 2Monetize plan to get the same level of service in terms of global tax compliance. This means I need to pay $0.60 per successful sale and 6%. So for a $1.99 app purchase which Apple would take $0.60 commission on, 2CheckOut would take $0.71 and I would still need to pay for my store front, this is not to mention other development expenses that are covered by the Apple Developer Program.
Of course, for me (unfortunately!), I don't make enough money to breach the Small Business Program (I wish I did), so Apple actually only charges me 15% commission, so my $1.99 app would cost me $0.30 in commission. If I took the cheapest 2CheckOut plan, it would still charge me $0.42 in commission, and on top of the other items above I mentioned I would still need to pay for, I don't get any analytics.
According to Apple, the average price of an app on their store is $4.90, even with this price, for someone selling on the App Store who revenues less than $1 million, their 15% commission would be $0.74 whereas with 2CheckOut they would be paying $0.90 for the equivalent payment service only.
Maybe contact their live chat and ask them how you can integrate their payment processing into your mobile app to support in-app purchases across multiple devices with account syncing. Of course, your users will need the ability to restore previous purchases so hopefully their API will allow you to query this, but I doubt its going to be Apple easy!
Apple doesn't give their developer tools away for free, nothing comes free in this world! They charge an annual developer subscription (which costs my independent business the same amount as the company I work for, and the same amount as they charge a company the size of Epic). They also take their commission, and with this revenue they are able to provide developer tools.