It's correct that you'll retain battery capacity better if you store it at 40-80%. But that recommendation is based on long-term storage.
It's not right to assume that also applies to in-use products, especially the way people use them. In fact you'd likely reduce battery life because you're putting the battery through a greater depth of discharge (DoD). Assume you let it run from 80% to 60%, you've just gone through a cycle of 20% discharge when you could have kept the DoD at 1%. Also, most modern laptops will run on power once charged, so it's not like an infinite number of 1% cycles.
Have manufacturers never considered this? If leaving them at 80% was better, they would've configured the adapters to do just that. Unless they really wanted to sell extra batteries, of course...
Letting the battery sit at 100% wears it out more than sitting at 40-80%.