tl;dr -- Evolutionary forces are far more complex and do not always select for strength over intelligence.

Evolution != bigger && stronger. If this were true, why aren't there still brontosaurus-sized creatures on land?

The truth is evolution selects the best adaptation for the specific ecosystem. Depending on the ecosystem, this could mean all sorts of adaptations, including more advanced intelligence and less advanced physical prowess, which is what happened with humans.

Your assertion that it takes too much time to evaluate intelligence is also not realistic. People spend years selecting mates, and there is nothing wrong with this. The average lifespan is 72-76 years and a good portion of that is fertile. We have plenty of time to select a mate and still have children to progress the genes.