If you didn't get that message by now and STILL check every crate, that one's on you. In particular since excessive looting is in no way required to finish the game just fine. You'll find plenty stuff else too.
I found a great ring in early game on some unmarked skeleton. The message I took from this event is that great stuff can be hidden anywhere. It took a few playthroughs to figure out that there is NO great item in any of the barrels or crates. Most players will do just one run. It's not reasonable to expect them to learn the logic behind loot placement within the first few hours, especially considering the large number of container types.
To be fair, I wouldn't put it past Larian to place a Sword of Infinite Power in some random barrel somewhere, just for the lulz.
Adding a billion crates and barrels is not great worldbuilding. There is nothing fantastical nor realistic about crates and barrels being placed everywhere. A crate is the least imaginative item ever. It has only two functions: waste space and time. We are so used to them in games that we no longer question the underlying logic behind their abundance.
