I think you may have misunderstood me - there's a distinction between what evolution rewards and what humans find rewarding. (This is getting hard to talk about because we're using "reward' to both describe the process used to steer a self-modifying intelligence in the first place and one of the processes that implements our human intelligence and motivations, and those are two very different things.)

The "rewarded behavior" selected by the original algorithm was directly tied to replication and survival.

Drug-stimulated reward centers fall in the "current behaviors that trigger the reward" category, not the original reward. Even when we self-stimulate our reward centers, the thing that we are stimulating isn't the thing that evolution directly "rewards".

Directly fulfilling the originally incentivized behavior isn't about food and sex - a direct way might, for example, be to insert human genomes into rapidly dividing, tough organisms and create tons and tons of them and send them to every planet they can survive on.

Similarly, an intelligence which arises out of a process set up to incentivize a certain set of behaviors will not necessarily target those incentives directly. It might go on to optimize completely unrelated things that only coincidentally target those values. That's the whole concern.

If an intelligence arises due to a process which creates things that cause us to press a big red "reward" button, the thing that eventually arises won't necessarily care about the reward button, won't necessarily care about the effects of the reward button on its processes, and indeed might completely disregard the reward button and all its downstream effects altogether... in the same way we don't terminally value spreading our genome at all.

Our neurological reward centers are a second layer of sophisticated incentivizing which emerged from the underlying process of incentivizing fitness.