There are lots of strange things that have, over the years, become gamer "collective wisdom." As with all such things, much of it is utterly wrong and based on misunderstandings and misapprehensions. One common one is that Advanced Dungeons & Dragons was an impenetrable maze of complexity. How anyone who actually played AD&D could say this eludes me, because, with the exception of psionics -- which I grant was indeed complex (and stupid) -- AD&D was, in nearly every respect, far less complex than, say, 3e either to prepare or run.
Were I to guess, I suspect that the myth of AD&D's complexity owes its origins in the Cult of the Universal Mechanic. AD&D has no single way to do anything. Sometimes high is good; sometimes is good. Sometimes you roll 1D20; other times, you roll D100. There are matrixes; there are tables. There are sub-systems galore, many of which have their own internal logic to them rather than building on existing systems. I deny none of this, but if this makes AD&D "complex," then what are we to call pretty much every game every published by FGU?
I ask this only partly in jest, because I simply can't get my mind around the idea that any gamer, in this post-3e world, could call AD&D complex with a straight face. Complex compared to OD&D perhaps, but otherwise? Not on your life.