Uller
Adventurer
The phrase 'you can't hide when belng observed' means what it does in the plain English meaning of the phrase, not in a DnD game mechanic sense.
As in: if you and I were in a room together and I watch you climb into a box, you are not (and can not) be hidden from me. I know exactly where you are.
Again...combat is not hide and seek. If you and I are the only people in the room you would be correct. But now put Bob the well trained fighter in the room. Bob has an ax. Bob wants to cut you into little pieces. There is a reasonable chance you will stop focusing on the cowardly little guy that ducked behind cover and focus on the more obvious threat. A well trained rogue knows how to take advantage of that and turn it into a more deadly attack. Once the rogue pops out and makes it clear he is also a big threat you might focus a bit more on what he is doing giving him disadvantage or preventing him from repeating his little stunt unless he gets clever..this is why we have DMs to look at the situation and make sense of it.