Earth’s gravity is pulling them in, even the ISS which is farther away has thrusters to correct itself from earths pull. While working they thrust themselves to stay in prbit. These are much closer and once they’re non-functioning they won’t be able to keep themselves from falling (“orbital decay”)

The satellite burning up on re-entry is part of what makes starlink such an expensive proposition. SpaceX can’t fix them, they can’t re-use solar panels or wiring harnesses. No part of the shell or shielding. It’s all just…gone.