They look like custom batteries. Why did they not use a standard battery like a 18350 or 16340 for the case? Other advantages too (easy to find alternative cha

They look like custom batteries. Why did they not use a standard battery like a 18350 or 16340 for the case? Other advantages too (easy to find alternative chargers and can keep a charged spare). I presume size constraints would make it difficult to find a commodity battery size for the earbuds?