We chat each other up. We write sonnets. And the positive feedback of sexual selection leads to fast evolution and great exaggeration of the chosen characteristic. But, of course, they were very successful for much longer than humans are likely to be. Although many animals have evolved quite a high level of intelligence, it is by no means a desirable feature for all species to possess.
Animals have a limited supply of energy, so they have to choose how it is allocated.
Two major uses of energy are brain power and reproduction. Humans and many other intelligent species have only a few offspring and look after them very carefully, which is partly thanks to intelligence. In contrast, insects, for example, tend to produce millions of eggs in the hope that a few will survive. This is very costly in terms of energy and any evolutionary trend towards having a larger brain would mean that they would have to reduce the number of eggs they produce.