DIY with hazmat can be something that nobody would insure the product if there is any class action. I am not going to be surprised if this is the reason many of the batteries are now sealed and cannot add water.
As to why batteries from Panasonic last longer. It could be because they use different grade of lead whereas the aftermarket just use the cheapest recycled lead without refining them first. If customers are not willing to pay 3x the cost for 3x the lifespan they wouldn't sell it. A new car probably wants to avoid false warranty claim by having better quality. The money is saved elsewhere.
Panasonic sold their battery division years ago. And the buyer does not use the name Panasonic re the batteries they make.