The skillfull programmer may accept liability when you give him a verification team with a few PhDs, the ability to withhold signoffs, flexible deadlines etc. etc. Few are willing or required to pay for that. So they get a mystery box with a 90% chance of crap.
"We need some software, ok let's get a big reputable company in to do it for us, we shouldn't get bogged down with all those horrible technical details"
They do understand that. They do not care.
I think for the courts the issue is a bit more subtle. The question is, who's job is it to prove that the other person is wrong ("burden of proof")? Should it be the job of the prosecutor to prove that Intel's processor produces the right answer when an ADD instruction is executed? Or should it be the job of the defendant to show that Intel's processor doesn't produce the right answer? What about proving that the compiler produced binaries which faithfully represent the algorithm? What about Excel?
In our normal life, if a computer is doing the wrong thing, we don't start by assuming a broken compiler; we start by assuming that the new, not-well-tested code is probably broken.
It seems that in the UK before the 90's, the burden of proof was always on the prosecutor to prove almost everything about the system, which is kind of ridiculous. So they passed a law trying to fix it, but messed it up the other way, putting the entire burden of proof on the defendant, without giving them any real way to disprove it. (I mean, shouldn't "discovery" at least mean I can inspect the source code?)
A more balanced law would say that widely-used software with extensive test suites can generally be assumed to be working properly; but that custom-purpose software needs at least some level of evidence that it's correct, and that defendants have a right to inspect any software that's used against them in court for defects.
Laypeople generally understand that software may crap the bed in the sense of "the system is down, please wait, then try again". But few people have experienced subtle changes in stored data.
A judge looking into his document cloud may be ready to see a "sorry, not available right now" notice, but doesn't expect that some sinister program is, in the background, silently editing texts of his judgments and pronouncing people guilty when he intended to free them etc.
The problem with the Horizon scandal is in this sinister manipulation of data. It may also have been done by Fujitsu people themselves, in order to cover some tracks and tamper with evidence. This is a very untypical failure mode.
Judges, salesman, and managers don't understand that.