It's simply not a easy problem to compute, even if it to our multibillion cell brain seems like a very easy task to solve in real life (and even in imagination).

Yes, Cyperium, you have got it exactly

The multibillion cell brain VERSUS the SIMPLE problem

Next x= 4x(1-x) is the now-classic example.

It was known for thousands of years yet only BELIEVED when computers came along.

It gives a series "too complicated for a mere formula"

The only way to find the nth term is to do all those nextx calcs!