I'm confused by your expert tradesman analogy. Surely the first step to hand-carving a beautiful wooden chess set is to throw away parts that have knots and cracks and won't carve well?
That's the point, you work with what is in front of you. That doesn't mean you'll have an identical outcome.
With a good teacher all but maybe the bottom few percent should at least be learning enough to be regarded as passing and most should be passing comfortably. A not so good teacher will go through the same basic motions using the same inputs but without that hard to quantify expert touch that comes from skill compounded with experience they will not produce the same result and they may not understand why because you don't know what you don't know. This generalizes to other skilled professions.
Don't get me wrong, I don't expect every college professor to be a great teacher. But the ones that aren't and don't want to be shouldn't be complaining that teaching is a chore and that some students are harder than others. That chore is what keeps the lights on.
Given infinite resources and any tool I want, even you or I could create a life size stone sculpture. Software and software run stone cutting machines make it easy.
But let's see what kind of a sculpture we can make with nothing but an 8 foot block of limestone and some chisels.