I don't, and I don't think it's particularly rational to assume that, to be honest. I strongly suspect the vast majority of people who run those books (probably a small minority of people who actually run D&D) actually do not run them as 5-8 encounters per day. I've played in plenty of groups, and podcasts and streams and so on pretty much universally reflect a lower number of encounters on the vast majority of days. It also only allows for extremely narrow "intense but not that intense"-type design of the adventuring day, and doesn't at all support how D&D and other RPGs have been played historically and continue to be played.
It's bad design.
EDIT - It's also circular logic. They designed the game for 5-8 encounters/day (which was seemingly already an assumption when they started playtesting, I note - I wonder where this came from?), and then playtested with weird little dungeon crawls as literally the only kind of adventure they were playtesting, and designed the entire game around 5-8 encounters/day and because they've done that, they release adventures with 5-8 encounters/day. So is it any surprise they remain satisfied with their own logic? Yeah, if you design for a ridiculously high number of encounters/day, and then release adventures that fit that, unlike the adventures humans actually write and run, sure, you're bound to be "satisfied". It's just totally circular logic, though.
A paranoid man might wonder if it they made it intentionally incompatible with how people normally run games, in order to make it so their own weirdly-designed adventures worked! Almost like DRM or something. But I suspect it's more likely to just be a weird fetish based on a dungeon obsession. It's downright hard to write a wilderness or urban adventure with that many meaningful, resource-draining encounters/day though.