Because of a boatload of correlations using a boatload of dating techniques, and a fundamental faith in reasoning by induction. Same way they know stars are x many billions of years old. What astronomical scientist actually nursemaided a star from birth to death?
Stasis in light of the Chicxulub event is a testament to non-change rather than change.
The diameter of the thread, and the size, density, arrangement and shape of the droplets, closely match those in webs made by modern orb-weaver or comb-footed spiders.
What killed the dinosaurs? Their sudden disappearance 65 million years ago, along with about 70 percent of all species then living on Earth, is known as the K-T event (Cretaceous-Tertiary Mass Extinction event).
But, of course, the answer will essentially be "it didn't change because it didn't change".