Term for being unable to see how hard something is because you already understand it
I'm not sure if this is an exact answer, but several topics come to my mind to describe this phenomena, mainly these ones:


Expertise (e.g. Expertise reversal effect),
Schemas and mindsets,
Availability heuristic (or other biases and fallacies that influence expectations).


It might depend on the aspect or scientific field from which you want to describe it. For example, there might also be some learning-based justifications from neuroscience perspective (e.g. reinforcement learning models for synaptic plasticity).