Explain Like I'm 5 (ELI5)

This is something I haven't given all that much thought about until recently, but it is quite infuriating. If you leave a steam deck or switch unplugged, the battery will drain pretty quickly. However, I remember the game boy and more particularly, my 3ds never dying if I had to step away for awhile. I knew I would have to charge it shortly after gaming once again, but it was never a big deal in the end. Looking it up and talking with friends, they came to the conclusion it has to do with the Lithium ion, but neither of us could explain well the why.