I think this comes up mostly because for normal cooking, you don't need a big fire - I can't even remember when was the last time I used more than 9 firewood + 9 branches (9 because that's the max you have to only press one key for), because for normal cooking, that's typically all you need and using more would be wasteful - and that doesn't give you enough embers to roast with. I don't often warm up the sauna either, because other than roleplaying, it doesn't have any purpose outside of some very rare occasions when you get the flu in winter (if it helps even then?).
After having tested this a few times based on the way I play, you need about 30 wood to have enough embers and you have to start either while the fire is still burning or right after (30 wood doesn't give you the ember graphics, but you can still roast) - if you start when the fire is out, you have about a 30 minute window during which to do it. But using up 30 wood almost never happens, so I would have to light up the fire especially with roasting turnips in mind; and this would be a bit pointless, because their nutritional value is so low. Basically, it comes down to needing so much firewood to have enough embers that it doesn't really come up in the normal course of play - even in winter I usually don't heat the fireplace for all that long in one go, because I'm out doing other things.