Why did civilization happen "now" vs. earlier?

There are lots of theories but the 0-th order term in the formula is climate stability. Everything else is an insignificant correction term. Without a stable climate agriculture is not possible and modern civilization is not possible without sedentary agriculture which is only possible if the climate is predictable and stable enough for farmers to have predictable yields and surplus.

Once there is food surplus it's possible to invest in cultural output and science to advance technology for exploiting environmental resources more efficiently with a lot less manual human labor. There is no free lunch though and the metabolic output of techno-industrial societies is poisonous to life (this includes the monkeys who figured out how to extract petroleum from the ground as a power source for their exploitation machines).

My guess: While the megafauna was around to provide low-hanging calories, civilization didn't arise because people felt no need for something resembling it. Once humans finished extinguishing most megafauna around 10 kyr ago[1], the small prey left couldn't providing enough calories for everybody, so people were forced into laborious agriculture to make ends meet, kickstarting civilization.

[1] https://www.nature.com/articles/s41467-023-43426-5

many times it takes 1000 years to lay the first brick and 10 years to finish the building.