Princeton’s Sam Wang has a nice summary of the perverse incentives that distort media coverage of the horse race:

It is not in the interest of individual pollsters or media organizations for you to have the most accurate possible picture of the horserace. Here is why.

Uncertainties such as the margin of error can be reduced by taking more samples…. The same is true for combining polls, with the added advantage of reducing the effects of methodological variation…

So why don’t more pollsters or media organizations aggregate polls?… Two forces encourage bad horserace reporting:

Competition among pollsters. It’s not in the interest of individual pollsters to say “average my results with the others.” It’s also not advantageous to collect a larger sample once the margin of error meets industry standards.

The hungry media beast. With news budgets on the decline, it’s costly to report real news. Why pay for investigative reporting when you can buy a poll and report the horserace? Within the area of poll reporting, market forces discourage high accuracy. For example, commissioning a survey of 4 times as many people would reduce uncertainty by a factor of two. But why pay 4 times as much for data that generate a lower likelihood of an apparent – and reportable – swing?