This is kinda not true, and also kinda true but for a different reason than you say.
Campaign-to-campaign differences are actually pretty big. Especially things like Hearth and Prosperity, which depends on war and raids and sieges, can affect prices a lot - and there's a big difference between a 350g/d brewery and a 500g/d oil press. And in the medium term, shifting control of castles can hugely shift the number of villages that bring goods to any given town, affecting inputs.
The problem; and it's the same reason people turn to third party analysis in order to just copy a meta; is that the player can't experiment.
Workshops are expensive to buy, pricey to change type, and sell for a pittance. You can only have a couple at once, especially at clan tier 3-4. And the only way to know what a potential workshop will bring in is to build it! So maybe in my campaign, an oil press in Jaculan will be >500g/d consistently, many years into the game. But maybe in your game it'll only be 200g/d. You have to go waste the gold, using one of your 4 or 5 slots, to find out - and then you have to repeat that for every workshop, times every city; and don't forget that the workshops you make can even affect the profits of the others. So the number of options is huge, the reward/penalty for choosing right/wrong are large, but the player is given extremely limited information and it's both costly and difficult to experiment. As a result, the meta is that players flock towards whatever is proven to work in any campaign, relying on outside information to make up for the lack of in-game information.
I don't come bringing a magic bullet solution for this problem, but if you want players to make more diverse and/or campaign-specific choices, you need to give them more in-game information. Whether that's being able to see AI workshop profits, or seeing an estimated profit when changing workshop type, or something else altogether, I dunno.