But these checks don't exist if you travel by road, train, or even by foot. Therefore I fail to understand why you feel they should be needed specifically for air travel?
Because you may not be the person boarding who checked the bag, and had passport / name noted, nothing to do with border crossings, only an attempt to thwart bombs in suitcase nutters who then don't fly.
Not sure if the previous comments about all bags been screened to the same level is relevant in Europe, I've only ever had a note left in my bag to advise that it has been opened leaving the USA, no European agency has ever bothered to check the same sort of contents that I routinely carry and seem to annoy the TSA.