I’m not a switch expert but can think of a reason why it might do this.
The system might show it’s battery level to you as 100% to 0% when it might actually be draining from 100% to 5%. That last 5% might be used as a sort of internal Uninterruptible Power System .
When the system boots up it might be doing some things where a power failure could have severe consequences like bricking the unit if the plug was pulled out and there was no battery.
The system might use some swap space like storage or have some key variables kept in RAM which needs to be written out to non-volatile memory before the chips are powered down.
For example let’s say it hibernates and it doesn’t or incorrectly writes the wrong instructions pointer address When the system poweres up, it might try and execute game data instead of instructions and not recover.
Nintendo wouldn’t want to handle heaps of complaints of bricked systems due to exceptional circumstances like a power outage if it let the switch play off mains alone.
That’s my theory, I could be wrong and I’m sorry it’s frustrating you.
I wasn’t trying to say that fault tolerant practices like journaling file systems wouldn’t be used, but to use an analogy, the system knows that when it’s low on power and tired to the point it needs a recharge, it can stop and lie down deliberately rather than keep running until it drops and maybe fall over and hit it’s head.