Because pleasure feels good and pain feels bad. Outside of those, there's no sufficient grounding for what should be sought or avoided.

You are missing out on a world of possibilities. I hope, at least, that you speak only for yourself? Because not everyone considers pleasure and pain to be in opposition and even among those of us that appreciate both to an extreme degree, not everyone considers them at the top of the list of experiences worth seeking.

On the other hand, something like wireheading is indeed a preferable state.

There are far more interesting things that can be done with wires[1] and electrical current than overloading a dopamine or opiate reward or pleasure center. How boring!

[1] For example, a tesla coil that uses electronic nipple clamps as a control circuit.