And I sincerely have to disagree with your disagreement. You're ignoring the cost of handling payment problems, refunds, tech support due to problems beyond your own control, the hassle for the customer to go to your site instead of the app store (I never do these days; not worth the effort, no matter how promising the app) and the cost of obtaining and maintaining your own certificates.
You definitely need infrastructure and maintenance, even if someone else hosts it. You need to build a web site, which is a complete waste of time you could instead use for development. You need to push this site, which requires further time and money spent.
And payment processing is a huge time and money sink. Every return processed takes effort, again better spent doing value add.
App stores are very high value. I haven't purchased a program outside of an app store in over a year for my personal use, and I don't foresee doing so again. There's no point dealing with individual sites and disparate payment systems when I can just get the app with a click, having the security of hassle free returns if something isn't to my liking, hassle free maintenance and one point access. Well, two point, really, since I use Steam and Mac App store, but that is still considerably better than the 20-odd points it would be if the developers insisted on their own sales and maintenance points.