Pick-A-Brick sounded like such a good idea on paper, until you realize that the economics of it end up being a handful of brick size/color combos and then bin after bin of minifig accessories.
I thought so too at first. My local toy store owner scoffed and called it “floor sweepings”. I think of that every time I visit a Lego Store and look down.
Through user groups, I’ve had access to direct ordering. I could order off the same list as Lego Store managers see. It truly was a random hodgepodge. Manager said sometimes they don’t even get to choose. Somewhere are warehouses of stockpiled parts, some of which are surplus after a set is retired.
For example, I chose a box of sand blue 1x2, and a box of 2x2 low slope red roof peaks. Imagine those languishing in a Pick a Brick wall. Myself, I had a lot of roof to build but I still have a thousand unused.
If Lego took consumer or store orders at random, it would complicate their planning which allocates production for sets, months in advance. Only the Model Shops got anything they asked for.
2x6: white
2x4: blue
2x2: grey
This makes interesting patterns, since you are more likely to use certain bricks in certain positions.