the real reason
Ronald G asked a bunch of specific questions and people mostly responded to those. But the answer to hte Base Questions of why are computers so difficult to use is as follows... They are and OPEN and ADAPTABLE SYSTEM. IF you designed one to just do one task - ex: Display Books on a tablet to be read, they would be simple. And those are easy to use.
If you designed one to just go on line and browse the internet and load up internet pages and do nothing else, it would be simple. But you can take a computer and set it up to do anything from design airplane parts in CAD and print the bluprints on a plotter, to write and publish a book, ready yo go to a the book printer, to log and control inventory in a giant corporation and automatically reorder items and track both retail customers and wholesale sources and all the money moving thru the corporation. OR run an online business. Or calculate physics problems. etc etc. As such it can be loaded up with a large number of different operating systems and programs and the computers themselves are not homogeneous the come in many types and sizes. Thus the interactions are almost infinite and therefore contain myriad unexpected permutations, interactions and anomalies... and THAT is why they are dificult to use.