Still, as a non-programmer, if everything is so "easy" isn't it surprising that there are very few actually working (free/low cost) JPEG recovery/repair programs?
A very minor corruption in the file and poof you can retrieve nothing.
Many years ago there was a single program[1] that could, in a very small subset of cases, "fix" corrupted JPEG files (manually/inteactively), now (AFAIK) there is almost only [2] and [3] and this (service) [4] that actually produce (in some cases) good results.
In my (admitted) ignorance on the matter, I would have expected that with progresses like increased computer power, AI, deep fakes or whatever the problem by now was - if not solved - at least solved in a number of subset of cases.
[1] https://directory.s2services.com/jpg-bmp.htm
[2] https://www.jpegmedic.com/tools/jpegmedic/ https://www.jpegmedic.com/tools/