What is the reason for hiding the slots on a timed node?

[LineageRazor]

You're right, your Gathering stat can be low enough that you don't uncover the item in one hit, and yes, items like Dark Matter Cluster, Unaspected Crystal, Mazlaya Greens, and Gregarious Worm are part of why this mechanic exists, as they may not even be present on the node. But obviously this design was abandoned, probably because it's dumb and annoying.

However I wouldn't hold your breath on a quick response even though we got a community response. Changing this mechanic would mean changing Endless Toil to something else, and, well, they've got more on their plate right now.

Actually, this mechanic was in place WELL before "variable location" items became a thing. The "clutter" was in place from the very beginning of ARR, while the first variable items weren't added until, I think, Patch 2.5, nearly two years later. While it's possible that they had planned for variable items from the beginning, I think it's far more likely that they put the clutter in place to give gatherers a feeling of discovery when they figure out where the treasure is, and to give them something to memorize in order to be a successful gathering class (much the same as DoW, DoM, and DoH classes need to memorize action rotations in order to be successful in their roles).

The problem is, memorizing times and slot positions isn't nearly as engaging as memorizing job rotations, and every other patch added MORE items that needed to be memorized. It's to the point where the individual who can keep track of where everything is is pretty rare, and that's even with SE trying to help out by having certain slots be "common" (mining hidden nodes are 3 or 6, more often than not, for instance).

Bad as Gathering is, Fishing is so much worse. A myriad of external resources is practically required for Fishing, and while they EVENTUALLY started adding some of that information in-game (time and weather requirements for certain fish), what they have is vague yes/no answers, and even those are unavailable for Big Fish.