I use a wireless Logitech mouse daily for work.
I have a mental disorder. I can easily forget to bring a cable with me so the battery will get much lower than the 10% low battery threshold. I can then KEEP FORGETTING to charge it. When I'm finally with a cable, it's nice to be able to use it plugged in.
I think this whole situation shouldn't be a WHY question, but a WHY NOT?
If this is the reason, it's very disappointing. They designed the mouse; they can make it smart enough to stop charging itself while still plugged in... Or is it because they want THEIR mouse on your desk to look 'clean' as often as possible?