In a well designed ecosystem there are many processes and "jobs" to do. If everyone made PhD's then no one would enjoy working as a cleaner.
For example, bacteria LOVE decomposing waste. The Beauty of the Design is that bacteria are very happy when they do it. They CHOOSE doing it. The same with dogs. Dog is very happy being a dog, even if we give that dog lots of love and attention. Because a dog cannot IMAGINE being anything else other than a dog - it is very happy.
What amazes me is that every creature, no matter how limited is its intellect and how initial is its stage of evolution - can feel, experience, express and communicate Love. How it is done? Is is a coincidence?
Hence, my conclusion is that different creatures have been "allocated" different conscious resources to make them HAPPY fulfilling their roles in Nature.
Another possible reason is that creatures that have limited intelligence enable humans to study the evolution of intellect. Did you know that the intelligence (comprehension and the ability to draw conclusions) of a clever monkey is comparable to that of a 8 week old baby? The baby may become a genious, but monkeys enable studies of intelligence itself.