>i.e. charge the battery to 100% (or something close) but power the laptop directly via the charger.

That's already what's happening. However that's not enough. Leaving the battery full is harmful in and of itself.

Are you sure? I had read elsewhere that it always goes through the battery, and a sibling comment says there is insufficient power from the charger alone (not really sure how that's possible either though).

Understood. So why hasn't Apple done something about this earlier? IOW, a setting that charges the battery to a maximum of 80% when plugged in?

Because you need to predict whether people want to use it plugged in or on a full charge afterwards.

>power the laptop directly via the charger

Recent macbooks can't run directly from the charger. The charger is not beefy enough to provide power during peak CPU/GPU usage.