> I just don't understand where the difficulty some people have in mind comes from so I'm trying to understand it better.
It's a pain in the ass from time to time, especially if you don't administer Linux boxes for a living. I've been running my own MTA for well over a decade, and have dealt with every one of the issues cited by the other people responding to you in this thread, as well as most cited by other people discussing this article in general, and a few (such as the advent of deliverability/spam-fighting tools like DKIM and DMARC) which I haven't seen anyone else mention. I haven't kept close track, but I'd say it's cost me altogether somewhere between one and two weeks of time over the years - and that doesn't count initial setup, because I did that back before you could just pick any of a dozen HOWTOs that'd take you through the whole process end-to-end.
For me, and apparently also for you, that tradeoff is worthwhile. (For me, not least because the amount of effort required has gone nearly to zero in the past few years. If that weren't true, I'm not sure I'd feel the same way.) For a lot of people, that tradeoff makes less sense than spending about the same, or a bit more, money, in order to have their mail infrastructure looked after by professionals who do it for a living. Granted, they have to deal with risks that we don't, like Google's habit of surprising its users with rather stupid UI changes, only some of which fail to last. But we have to deal with risks that they don't, too. We prefer control to convenience and are willing to spend time and effort to get the result we want, and that's okay. Others prefer convenience to control and would rather spend money to get the result they want, and that's okay, too. Different people have different needs.
If that's still confusing, I don't really know what to tell you. Sorry.