I would compare this to asking, "Why don't software developers think about bugs?" They do, and they attempt to handle them, but it's almost impossible to solve all such problems in programs (tests) of non-trivial size.
I used to write tests all the time in my subjects and my tests could not be gamed. I would have half multiple choice questions drawn from every area we covered in the semester and half essay questions where the students had to integrate what they learned into a coherent answer. My students would ask me what they had to learn for the final exam and I would always say everything - if it wasn’t going to be examined there was no point teaching it because the students won’t both learning it :)
Though, with enough resources, time, and effort, I think almost any program one would want in practice could be written with a formal proof of correctness. (I won't say every, because that runs up against halting problem / incompleteness things, but I think for almost all that you would actually want)
It's just that there isn't enough demand, and time, for the programs to be proven correct.
If there was a program that we could all stand to wait for a very long time for (maybe even a few generations?), and it was important enough that it be completely free of bugs, that could be done, I think. I'm not sure what it would be that important though. Starship stuff?