Why do satellites launched into low$-$Earth orbits not remain there indefinitely?

they do not have enough speed to fall freely around the Earth

they tend to collide with other spacecraft and spacecraft fragments

this is an unsolved problem, but a NASA committee has been assigned to work on it

they lose speed due to friction with the upper layers of the Earth's atmosphere

they run out of fuel, and without fuel, all satellites, no matter what their orbit is$,$ must fall