A computer opponent poses no challenge, there are always exploits and once you work out how they behave, every game will be a near mirror image of the last.
A human opponent, when matched to your ability, will pose much more of a challenge, will, if they have any sense, adapt to the plays, are always unpredictable and what is more satisfying than someone raging because you are doing so well

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Computers cheat far more than human opponents do. It is a tiny fraction of online gamers using cheats, whereas in certain games, the computer is given an advantage right from the get go to try and make the game harder. The computer not only cheats but is designed to. It still doesn't make it hard to beat them once you have worked out how they are programmed to behave, but it is still cheating.
Plus if you consider co-op, it is always more of a laugh to play a game with friends, or even random people that can have fun as well, rather than just be there. Computers also tend to have huge problems with co-op, like bad pathing or making stupid decisions