In what way do you mean iterations - what's your model for iteration as a mechanism for iterating intelligence? Do you mean that intelligence improves with every generation?
I mean iterating to build the technology required to visit the moon, starting way back with fire and a stick. No different than an LLM iterating to build software. As you said human fluid intelligence has likely not changed significantly.
Because the requisite amount of iterations for their/our level of intelligence had not yet occurred. After a 100 billion lifetimes or so we got there.
Presumably entities with greater intelligence would require fewer iterations.