Why doesn't a jet's anti-collision system simply stop a runway crash?

Answer: It is built to track other aircraft, not ground vehicles

It only switches on during daylight hours — Wrong. These systems do not switch off just because it is dark. Their real limitation is scope: they are not universal magic shields for every object that could appear on a runway.

It is built to track other aircraft, not ground vehicles ✓ — Correct! Many famous anti-collision tools on airliners, such as TCAS-style systems, are built mainly for aircraft-to-aircraft conflicts. A runway crash with a ground vehicle is a different class of problem. That kind of safety depends much more on airport surface tracking, controller warnings, stop bars, procedures, and timing before the aircraft reaches the obstacle.

It is wired to steer airport fire trucks directly — Wrong. A jet's onboard anti-collision system does not command airport fire trucks. Ground vehicles are managed through airport and tower systems. That is why an airport can be technologically advanced and still need separate ground-side protections against runway incursions.