Try sticking your Sony or Samsung under the rear tyre of a bogged down vehicle in the middle of a field to gain some traction to get it out...
We had a field engineer literally stuck in his field. He stuck his Husky under a wheel of his Mercedes G Wagon and managed to get himself unstuck. After driving out of the field, he walked back, dug out the Husky, washed it off in a near-by stream and carried on working with it.
I'm guessing the Sony or Samsung won't stand up to that sort of abuse. But if you want to test it and prove me wrong, go ahead.
Also the IP65 testing is relatively cheap. All the additional rugged bits on the tablet for the rest of the certifications isn't cheap (well the raw materials are, but the design effort to produce them isn't) and then there are the prototypes that get destroyed during development and then the expensive certification testing.
We make our teminals to IP65 and IP69K, that is expensive enough. IP65 is enough for most people, the IP69K is great for our industry (food production, where they are cleaned at the end of a shift with a high pressure stema jet). Mostly they just want to ensure that blood doesn't get into the works and that the cleaning liquid that follows it.
Out in the field you generally don't need a tablet that can spend an hour at 1M under water. IP65 is enough for most purposes (generally they go above and beyond IP65, but don't reach the breath holding qualities of IP67 and IP68).
Our terminal held for over 24 hours at 95cm, but didn't survive the hour at 1M (the only tank we could get on short notice for internal testing was a couple of cm too short). The extra couple of CM make a big difference. That said, we are hopeful our modified design will get IP68 next time around. In essence, it means the terminal is IP65; even though it can spend a whole day submerged in water, it doesn't rate as IP67 and waterproof.