Why do electronic devices like laptops and televisions use electricity when they're plugged in but no one's using them?

It's called "phantom energy."

All of the things in our home or car that are plugged in can be using energy, even when not in use or turned off. Phantom energy can be scary for the energy bill, but no actual phantoms or ghosts are involved. Our devices keep using energy in this state because all of those electronics are standing by, ready to jump back to life when you need them. In fact, your TV’s phantom (or standby) usage can make up to 10% of its total power usage.