This assumes you have all the source code for everything available, the person to do it ready, and that no compiler errors anywhere have crept in through the many revisions of the SDK. None of these assumptions hold for iOS ports that sold poorly (as most did), that were done by some team (likely defunct) as a one-off, that probably used a bunch of middleware they didn't own. It's just not worth it, otherwise it would've happened.
The transition to 64-bit started in 2013 with iOS 7, 5 years after the App Store launched and included a 64-bit compiler in Xcode 5 that year.
Not long after, 64-bit was set to default for new projects in Xcode.
That was the point developers should have made the transition.
Instead, we saw that most developers didn’t bother, and by the time 2015 rolled around, and 64-bit became mandatory, many had still published 32-bit apps in the two preceding years.
In other words, most of the abandonware was made in the two years leading up to the 64-bit requirement despite ample warnings.
Now, you have me believe that they didn’t have their source code lying around somewhere? I’m sorry, but that’s just not believable.
So, the source code was available. How about the people who could do it? Many makers of abandoned apps and games happily continued publishing new 64-bit apps and games after the transition (e.g., Capcom), so I think it’s safe to say that the people were available.
Now, what about the difficulty of porting it to 64-bit? I can’t find an authoritative source that describes it objectively, so I’ll just have to throw in my own anecdotes.
I’ve done about 15 ports for various people with apps varying from your garden variety app to some games.
As I stated above, in most cases, it was as easy as recompiling it. In a few instances, Xcode would throw up some warnings and errors, but those were all of the “you need to do this” variety, and working through them didn’t take me (a solo dev) more than a day.
Games were a little more involved in some cases because I needed to audit pointer usage and optimize memory usage, but even so, in all cases, I had a build up and running within a week.
Would all of this been harder had this been abandonware from two decades ago that nobody had touched since? Probably.
But specifically, in the case of the iOS transition to 64-bit, it mostly boiled down to “just recompile it, bro,” as you so aptly stated in part because they were, by definition, relatively recent projects.
So to me, the ones that didn’t do it didn’t bother because it wasn’t opportune to them, rather than out of a lack of ability to do it.