The computer is harder than a proper opponent. It knows what you'll do before you do.
Slightly wrong, it knows what you pressed the very moment it's pressed and it instantly counters it without button delay
That's what SF does and why people want AI's to have button delay so when playing the AI on hard/very hard, it's not counterpicking your inputs.
To be more specific.
When I press X the computer immediately knows and does an input but unlike a human when it inputs it has no lag to it. Humans have to physically press it and the motions needed, and so on, but the AI just instantly does the input.
So in SF4, for example, if the AI sees I jump, it immediatly does a FDF.M faster than any human would.