I assume that not everyone leaves their router on 24/7 and that some still only switch the router on when they know they are going to use it. The older generation in particular (talking as a 54 year-old, lol) tend to see the internet as something they do occasionally rather than a continuous part of their life. If there are enough people who only switch their router on two or three times a day they might represent a worthwhile saving to the ISP.
Turning an ADSL router on and off is not recommended, it is seen as line instability and the equipment in the exchange is likely to lower your sync speed to compensate. Best to leave it on all the time.
If you have a VDSL connection turning your router on and if is completely stupidity as the equipment in the cabinets is almost certain to lower your sync speed in response to what it would see as line instability. It would only be done by someone who is ignorant of the underlying technology.
Changes with FTTP technologies but frankly your ISP won't thank you for it.
I very much doubt any significant proportion of the population only turns their router on when needed. I don't and I am only a few years younger than yourself. My mother does not and is in her 70's.