Battery life and size of player is something that companies need to make a considered decision on. It could have a larger battery for longer times, but how much time do you need in reality?
In one day's listening, are you going to play it for even eight hours? If you listen to two albums a day, that would be 5 days.
You can charge it any time, so when I get home, I just plug in overnight. I've never been without power.
Having higher power allows a much wider choice of headphones and it drives them very nicely.
Having a battery life of 40+ hours would perhaps lead to you actually forgetting to charge it and leaving it for longer periods and in any case, having low power output is not the best for good dynamic range.
So there are reasons and tbh, it is a great sounding dap at a brilliant pric