It's not that baffling. It let them keep the exact same form factor as the previous non-rechargeable Magic Mouse, and the mouse can go long enough between charges and charges so fast that they knew in practice 99.99% of Mac users would not have a problem with it.
It charges to full in a couple hours and lasts at least a month on a full charge so almost everyone has plenty of opportunities to charge it at times when they aren't using the computer.
If you do let it get low enough that it might die it only takes a couple minutes of charging to get enough charge to last 8 hours. Start charging and go get a drink of water or visit the bathroom and when you get back it is ready for the rest of the work day.
That said, from what I've read there either is no warning when power gets low or it doesn't always work. With it only needing charging every month or so I can someone forgetting. It is possible to make a script to check it and give an alert if it is low, and make a cron job run that script [1]. That really should be built in.
[1] https://dev.to/samselfridge/magic-mouse-low-battery-alert-4m...