Why is the use of electricity prohibited on Shabbat?

Hello, Your Honor,
I am not asking this question to be defiant, but rather out of a genuine desire to know.
I read that the decision not to use electricity stemmed from the same period when electricity and the light bulb were invented. The incandescent bulb had two strands and would actually create a spark. There is no such thing in an LED bulb at all, so no fire or even a spark would ignite. What’s more, it is permissible to use electrical appliances that have been left on since before Shabbat, such as a refrigerator or plasma TVs in synagogues, so a cellular device is also kosher, even if it runs on batteries and is always on, and even if it is not on, you turn it on. What’s the problem at all? Where does this come from?