Do people really think pixel hunting or arbitrarily stumbling upon critical items in unrelated areas engages their brains?
How? Just, how?
A lot of people seem to like it, but it stinks of Stockholm syndrome to me...
Well... anything that creates any kind of stimulus engages our brains.
It's just whether we appreciate the stimulus or not.
As an example I'll give you: the acorns in BG2:SoA. If you find them the first time you play the game, and happen to carry them around with you until the Windspear Hills, there is a sense of accomplishment when you can finish the quest. If not, maybe you want to improve your experience by finding them/carrying them around the next time you play the game.
Pixel hunting is a bit different, but there should be the occasional non-combat barriers towards getting further into a dungeon etc. (mirror dungeon was nice). Maybe it's best when there's a bit of variation in how this works, like different types of puzzles, but if you had a 'secret' lair you'd want to keep it secret right. It's also a quirk of the game that it breaks immersion and the fourth wall a lot (earth statue as a prime example, which I personally found hilarious).
At the end of the day, Larian created the game they wanted to, and people will discuss whether or not they like this particular feature or not. It's only really if there is a mass consensus on a particular feature that it becomes less of a debate and more of a dubious design decision (Tenebrium, difficulty post cyseal are the things that spring to mind, and I'm not even sure there is consensus on the latter).
