Indeed they do. Many of those families and dependents have individual bedrooms. During the day, when they're not sleeping in it, those rooms have sat empty because the person has been at school or the office.
Same argument as before, I guess. I could sit in there working away quite happily. No extra room needed.
Sure, it's not the case for everyone, and some people share bedrooms, but the blanket statement that "working from home essentially requires permanent rental of a room on your house you might otherwise use for something else, or not need in the first place" is a massive generalisation that doesn't apply to many households in which people have bedrooms and in which there is a lounge.
Sure, add a desk, change the furniture layout. I'm saving 25% of my rent by not having to commute. It's a two-bed flat; between us, we have 50% of the rent to spend again each month on furniture, rearranging things, or just renting a place with two extra rooms. Not that we need them, with three rooms available to work in during the day (two bedrooms and a lounge). My experience is not uncommon and in no way do we essentially need more rooms to work in.