Energy efficiency has been completely deprioritized.

Here, have a video from the HP Handheld Conference 2011, in which the program manager for the HP 12C calculator talks about battery life (jump to 4:26 if that got removed):

With two (presumably LR44) button cells - so about 3 V and 110 mAh nominal - they found that 3-6 months worst-case battery life was unacceptable, so they redesigned the case to fit a third cell in there. And, with that third cell, there’s plenty of anecdotes of calculators lasting over a decade on a set of cells.

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Meanwhile today, 3-6 hours worst-case battery life on a 3.8 V, 3000+ mAh nominal battery is considered perfectly acceptable on our modern phones - sure, they do a lot more, but still, come on.