But lots of these companies actually do that. Taking just the example of Spotify, they do run their own infrastructure, credit card processing, marketing, advertising, sales tax, websites, CDN, etc. The issue really is that Apple forces you to use their infrastructure. What if all of those companies would be more than happy to bear the burden of those costs, but they literally can't do so because, in order to reach millions of people on iOS, they have to use and pay for the Apple infrastructure? Or, more precisely, what if they already do bear the burden of those costs, and now they have to pay a 30% or 15% royalty for services they really don't need, which becomes a redundant expense? What if, instead of paying 30% or 15% of your revenue, you just paid Apple a fair price to be listed on the App Store and to cover server/infrastructure costs?
You could simply say "ok, screw Apple and their customers" but given the market share of iOS, you're basically cutting off your arm to save your face at that point. If you want to actually get your product in front of all those Apple users, you are forced to use Apple's systems, and pay for them accordingly. It's probably a fair statement that there are few apps or services that would actually cause an iOS user to abandon the Apple platform entirely. Therefore it's also reasonable to say that it is a monopoly for those specific customers.
Imagine if, to distribute an album on CD back in the 2000s, you had to pay Philips 30% of your album sales revenue, because come on, what about all that R&D to develop the CD? Even if you were willing to build your own CD pressing plant, do your own advertising and marketing and distribution, etc. you still had to pay 30% to Philips? And if you didn't want to do that, you'd be unable to sell your album to the millions of people using CD players. Ok, maybe 30% isn't a fair comparison, but assume even 10%. I could actually be completely wrong, but I don't think that CD manufacturers were required to pay royalties based on sales revenue. Maybe there would be a flat rate per disc, but Apple isn't doing a flat rate per download or per purchase, they're doing a percentage.