Different traits work to fill different niches. We're smarter than lions, but would you want to be dropped naked and unarmed on the savannah near a pack of these cats?
Other species have hit pretty much optimal designs for what they do. A shark is dumb as dirt, but it would be difficult to improve on the design. What environmental pressures would serve to make sharks evolve higher intelligence? Would being smarter really make a shark a better hunter? It might, but the tradeoff in energy required to feed its larger brain might be an evolutionary drawback.
Our ancestors developed intelligence because it was what they needed to survive. Before the increase in intelligence, hominid species' were not all that succesful and came close to extinction on several occasions.
Even if I am to accept your argument (which has obvious and numerous holes), I must contend that only ONE species has evolved to sentience.
The cheetah is the fastest land animal, of the thousands of species out there. The elephant is the largest land animal, of the thousands of species out there. Similarly, we are the smartest species in existence. However, that's just a matter of degree. Elephants, chimps, gorillas, whales and dolphins are also intelligent, just not as intelligent as humans.
Since a leopard is slower than a cheetah, is that an argument against evolution, in your opinion?
Survival. Is that not the basis for all Darwinistic evolution??? Man could exterminate every horse in the world and they would have no (or very little) power to refuse...
There is no master plan when it comes to survival. Species respond to external stimuli and evolve accordingly. Sometimes, those stimuli occur too rapidly for species to adapt, so they go extinct.
At this time, though, there is no environmental pressure for, say, hippos to evolve a higher degree of intelligence.
And if I wanted a smarter one, just what 'pressure' would have to BE applied?