The standard is mine and I follow it. The same way I find absurd not to do it, you find it unreasonable to do it.
I find the expectation that things are not stolen (if unsupervised in public places) strange considering the huge amount of inequalities in wealth around even in civilized countries. I do not agree with the idea of stealing, thiefs should be punished, but expecting everybody "to behave" given the situation seems unrealistic to me.
That does not mean that I think that things are stolen 100% of the time. I have a friend that forgot a laptop on a bus (Netherlands) and the driver found it at the end of the line and gave it to lost objects so my friend got it back.
If you find it absurd how 99% of people act on long-distance trains, I don't know what to tell you.
I don't think I've ever seen a human being do that before on a train. Not to go to the toilet, nor to grab a coffee in another car.
You can't be paranoid about everything. My friend in France had put his laptop back into his bag where it wasn't visible and assumed that was good enough, but someone must have seen him do it and just took the whole bag.
You are applying a totally unreasonable standard, to suppose that the thefts were due to unreasonable carelessness. What, do you think someone should take their large luggage into the bathroom too, every time they need to pee?
Talk about victim-blaming.