What the system was built around (at least, according to the 2014 DMG) was "Assuming typical adventuring conditions and average luck, most adventuring parties can handle about six or eight medium or hard encounters in a day. If the adventure has more easy encounters, the adventurers can handle more. If it has more deadly encounters, they can handle fewer."Yeah, because they didn’t want to do that many encounters. But that is what the system was built around.
Obviously, that means that not every day needs to have six or eight encounters, so long as you keep an eye on the difficulty. Hell, I bet that why most DMs ran fewer encounters, and they tried to make them harder. The problem is that what the 2014 DMG called the various encounter difficulties didn't align well with how most DMs understood the words.