Have you looked at how much stuff is packed in those AirPods? There’s certainly not enough room for a screwable cap and a consumer-safe lithium battery, while retaining rigidity and weatherproofing.

Bedsides, how many AirPods are likely to be dying from poor battery life compared to being lost, damaged or upgraded?

If this did happen, I suspect there would be more instances of people losing the battery cap, or throwing it away because of undiagnosed battery terminal faults, or of children dying because they swallowed the impossibly small pill shaped battery...

You could screw something on there. Just make the part that hangs outside your ear 2 mm thicker. It wouldn't be in your ear. Apple wants those things to fail, battery life to run out and not be replaceable. It's completely obvious.