I've spent a while searching for evidence, and as best I can tell the average table has 3 combats a day
4 isn't unusual and is more common than 2, but in terms of actual, practical gameplay the average seems to hover just above 3. Anecdotally, that matches my own experience going back beyond even 5e, there are always exceptions, some days have one or none and some days have lots. But overall, 3 or so matches what I've seen.
So where is the 6-8 per day thing coming from? They know very few people play like that, and it would be absurd to expect people to randomly change their narrative style to double the number of fights. So like... why is that number treated as being relevant, by anyone?