Because the cost of finding out who the most competent legislators are is much higher than the benefit I get from casting my vote, which has a one in several million impact on the outcome of the election.

It's why most people vote for who is the most recognizable, or who is the tallest, or who had the most ads on TV, or who their family and friends vote for. The cost of informed voting is much higher than the benefit. It's just one example of the market failure of the political system. In other words, everyone does what's in their own self-interest, and we're all worse off as a result.