I have an account on Seasoned Advice, and it never even occurred to me I could ask about coffee there (i.e. as Robert described). They're in very different mental categories for me, but as a result of them also being in very different categories in the real world.
Consider that the Seasoned Advice's on-topic help page says that it's about kitchen work, meal preparation, picking ingredients, and food storage. It's built for and marketed toward people who cook, wear chef hats and use seasonings:
None of that is really associated with coffee culture. We're concerned with different things, like espresso machines and water distillers, coffee grinders, bean roasters, and milk steaming.
Consider that in addition to Coffee, there are also beta sites or proposals for beer, wine, preparation of both, and other alcohols and what to do with
them — those people aren't going to Seasoned Advice either. This is because a chef, a bartender, and a barista don't expect the other two to be able to do the same job as them — except by coincidence, like if the bartender is also a good cook.
In the Venn diagram of coffee culture and cooking culture, there is a great deal of overlap, but also enough specialisation and unique elements to justify zooming in on the coffee bits in another site — just like Seasoned Advice zoomed in on the cooking portions of that Venn diagram.
So, that's basically it. We have a site for coffee drinkers because coffee drinkers want a site that is for them, and the site for them isn't the site that was made for chefs.
Chefs who are also coffee drinkers are welcome to join Coffee.SE for coffee matters.