Question

Why is compression a "hard problem" for computers?

Answer 1

Compression is hard because it is hard to determine how much space you will need on your computer and it is hard to make the space you use optimal or satisfactory. Trying to make the space optimal also wastes time and it is hard to tell when to stop optimizing. Compression deals with using less space or bits on a computer to represent information.

Answer 2

Compression is a method for compressing the original information which means using fewer bits to represent the original information.
Compression can be a "hard problem" for computers because it is difficult to know whether or not the compression you've found is optimal .
It is not easy to find out when to stop compression and to determine the compression you made is enough.