This is a
A tawny pelt stretched toward the sky
It reached ground long after it died
Traditionally pizza crusts ("tawny pelt") are tossed in the air to stretch them
My zestful hearts were next to fall
Our blood spread thin amidst it all
Tomato sauce is definitely the blood. The zestful hearts could be tomato pieces or maybe pepperoni slices. (The more I look at this, the "hearts" become unclear to me. Artichoke hearts are also possible pizza toppings.)
Our culture was the last to go
Torn apart before they zapped the whole
Shredded mozzarella ("culture" refers to cheese cultures, and it is "torn apart") is added before the pizza is baked quickly at high temperatures ("zapped")
Don't ask who, instead ask why
And find the answer simple as pie
Pizza is often called a "pie" or "pizza pie"
edited to add: I forgot to mention the title.
What the rock glimpsed
Pizzas are often baked on a pizza stone