It's not so much life experiences, but prior knowledge and familiarity.
Taking mathematics as the operative example. A lot of deep theorems are trivial when you fully understand all relevant definitions. These proves cannot be stated 'in simple terms' because it took a stack of non-simple definitions to even formulate such a theorem.
Explaining the entirety of the stack gets hard when the stack gets deep. For example, a lot of math gets really hard to explain when the concept of the real numbers being 'uncountably infinite' whilst the rationals are 'countably infinite' isn't obvious.
Taking mathematics as the operative example. A lot of deep theorems are trivial when you fully understand all relevant definitions. These proves cannot be stated 'in simple terms' because it took a stack of non-simple definitions to even formulate such a theorem.
Explaining the entirety of the stack gets hard when the stack gets deep. For example, a lot of math gets really hard to explain when the concept of the real numbers being 'uncountably infinite' whilst the rationals are 'countably infinite' isn't obvious.