They're not "designed" that way.
You do realize even though there may not be audio going in/out doesn't mean they're not in use? There are processes going on in the background like listening for Siri, or the Find MY services as stated earlier. This will cause battery discharge because it is in use doing as intended. There is a difference between discharge and degradation, which is a decrease in battery performance, and any battery will do this over time. Nothing about Airpods battery capacity/capability is unusual, unnatural, as you say awful in design. Those people who get 2-3 hours of use after 2 years is still very acceptable usage.