Because by "works" you mean "I tried it out a bunch of times and nothing bad happened, so must be production ready", because that's what you do when you build websites and desktop CRUD apps and nobody ever died cause the web server choked under load or the web page rendered a bit funny or the request took a full quarter second because the GC kicked off as a wave of requests came in.
And then one day a one-in-ten-million event that never would've surfaced during testing does happen and the OS crashes or the interpreter hangs or some non-determinism causes a period of non-responsiveness or a bit gets mangled because you don't have any redundancies and solar rays are thing or ... and someone dies. And of course that didn't happen in testing, because all of those things are rare possibilities.
And then it happens again.
And again.
And then you're in court being sued for millions. And a bunch of industry experts who do build their systems with redundancies and do design their systems with a safety-first mindset come to the stand and rip you apart for not taking even the most basic precautions. And when it comes out just how many best practices you completely ignored, you're mostly just hoping beyond hope that all of the legal problems stay on the civil side of the civil/criminal divide.
Or, to say it in a sentence, "because the sort of exceptional circumstances that safety-critical software needs to handle with grace are very difficult or impossible to account for with a desktop machine running interpreted code on top of Linux."