I'll probably get flamed for this, but I think it's a legitimate point so I'll make it anyway.
A lot of our judgments about others has to do not only with their recognized level of expertise, but also our "sizing up" of how intelligent that person is in the first place. I've had doctors who were clearly not up to intellectual standards, and this was usually bore out in their advice (e.g. Your shoulder hurts? You should just never bench press again, and it'll clear up.).
I think that, at least among the (formally) educated population, there might be a bias against trainers and such as either "meatheads" or being folks who weren't intelligent enough to do something else with their lives. As such, the thinking might be that while they may have a basic template that's useful for making progress (as evidenced by their own), perhaps they haven't thought about all the possibilities that their basic template might offer.
So, for example, I see CT post a routine and think to myself, "Yeah, that routine looks pretty reasonable given its underlying principles. Perhaps if I [insert tweaks], which also align with those principles, then I might get better results." I justify this to myself by reasoning that CT definitely knows more than I about training principles and such, but I fancy myself to be much more clever thinker than CT.
One of the big problems is that a large part of the population isn't more clever than CT, but I'd imagine that only a small percentage of the population realize themselves to lack that level of cleverness. As such, their additions are usually somewhere between superfluous and stupid. Moreover, the initial justification about cleverness isn't warranted, as it assumes that there is an increasing marginal gain from cleverness being injected into a program when, as you say, a lot of success is based upon very simple principles.
So, I would say that most of it isn't a lack of hard work so much as actual stupidity masked by perceived intelligence. Sorta like how so many folks think they have "perfect form" when squatting, benching, etc.