You get what you pay for.

Tool batteries as a general rule don't do anything fancy internally, so they are easily substituted. I suspect it's mostly a matter of the main buyers being tradies/workers who use them to make money and thus don't care that much about the expense of the batteries - they probably aren't replaced that often.

Would be nice if there was a 3rd-party seller known to use quality cells though, rather than unknown off brands.