I'm not an expert on astrophysics, or millitary technology. Luckily, I have a friend who is an expert on both and has super-hard science fiction world building as his hobby. So I'm just going to quote him on why this isn't as big a deal as people think.

A related issue is the use of spacecraft as weapons against ground targets. This is unlikely, to say the least. As mentioned above, a sectional density of 10 tons/m2 is required to make it through the atmosphere with a reasonable amount of velocity remaining. While common ships might reach this threshold, they are almost certain to fail structurally long before impact. When they break up, the pieces are unlikely to retain sufficient sectional density to do damage. The result is a high-altitude airburst, which, given expected vessel sizes, is unlikely to do significant damage. It has even been suggested that vessels be intentionally designed to break up on atmospheric entry to reduce the risk to people on the ground.

This is from a recent paper meant as an introduction the theoretical space warfare community -of course that is a thing- consensus with an emphasis on what are called plausible mid-future scenarios.