The longer production chain also requires more resources spent along the way, since pops have upkeep.
Factor in pop CG upkeep. Yes, brain drone burns 3 minerals, but in regular empire researcher himself consumes 0.5 or maybe even 1.0 CG and same for artisan.
6 minerals, actually. It's 6:2 which 'reduces' down to a 3:1
Adding in general pop upkeep makes it a little better (and a lot more complicated to work out thanks to habitability modifiers) but it still looks like a net loss mineral wise. Unless you're spamming your production on <20% habitability planets and never improving that.
Also artisans themselves have CG upkeep and non-slave workers have CG upkeep too. Plus most jobs creating amenities for regular empires use CG.
Looking through the job list, there are only 3 jobs that create amenities and cost CG (pop upkeep notwithstanding); and all three are giving you something on top of the amenities.
Entertainer: 2 unity + 10 amenities for 1CG. So... 0.1 CG per amenity if we call the unity a token freebie. This is practically free.
Medical Worker: +5 Pop Growth speed, +5 Amenities for 1CG. You're probably running that one for the pop growth boost more than the amenities, so we might as well call them a freebie.
Priest: +2 SocResearch, +3 Unity, +5 amenities for 2 CG. This one's hard to evaluate; worst rate if you're running them primarily for amenities, but I presume one would actually be running them for the whole combination.
Everything else is free, other than the pop upkeep itself. Hell, with the Consumer Benefits policy Clerks actually net you CGs on anything other than Utopian or Chemical Bliss living standards (habitability multipliers notwithstanding.)
That said, I don't really know what proportions of any of those jobs a well-tuned standard or megacorp empire is going to be running. You seem to be taking Temples for granted even more heavily than you do ecus, though, so I imagine you're running a lot of the priests.