I think the misconception might stem from the fact that
* IKEA instructions for even very simple things like shelves are often up to 20 pages long. A lot of steps are very simple, but there are lots of them.
* IKEA furniture parts often have many markings on them to make them unique. This is necessary for reuse across products, but also identifiability for above-mentioned instructions.
The shelf I recently assembled had none of these "problems". In fact, it only had 2 types of structural metal bars that were identical except for length, and boards to go on top of those bars.
The instructions were 2 pages of text. Full paragraphs worth of text. It included sentences like "Connect the vertical corner pieces such that the second-to-last lower peg of the top piece connects to the topmost peg of the lower piece".
That one sentence takes around 10 seconds to parse, 30 seconds to visualize and then 1-2 minutes to lay out with the physical pieces in front of you.
I would estimate around 80% of that time is waste, compared to just printing those instructions using one IKEA-style diagram.
If I'm unsure which screw they really mean here, which orientation I should have that piece in etc., I can just look really closely at the drawing and identify exactly what they mean. See the different layers of depth and how they cover each other to see the exact order of things. See the the exact drill holes or other marks to disambiguate things.
Maybe it's just that some people think more in words others more in images and prefer instructions of the corresponding type.
And I do think IKEA are close to Lego in this.
It's everything from ensuring you illustrate dimensions of different parts in a predictable way; choices of angles, and accounting for the fact people are awful at mentally adjusting for movements in 3d, so if you don't make it easy to align the drawings with "reality" things instantly get harder. Getting shading right. Not making the steps jump too quickly.
Ugh. Assembling some (most?) non-IKEA flat pack furniture is so terrible. They often try to use the same pictures without word format, but without doing the drafting to back it up.
The one IKEA experience I had that was truly awful was a two-box set that had multiple manufacturers; we ended up with box 1 from one and box 2 from the other, but both had pretty much the same content. Getting all of the first box back into the box after having built that was very demotivating. I now know to double check that all of the numbers match, not just the big number when getting a two box set.