They'd rather increase the top line (revenue) by growing the market. That means keeping it as cheap as possible for advertisers to use, while still covering their costs.

The idea is to get media buyers used to a given rate per video view, then use increases in YouTube usage to increase the total amount of money they bring in (revenue), and use economies of scale and technology change to drive costs down.

The long game for them is to build the perfect video advertising platform on YouTube, and then allow other video platforms to use it (in the same way a huge number of websites use AdSense/DoubleClick).

There's a lot of money in TV advertising. Google wants that money, and they can afford to wait another 10 years for the market to develop enough for it to happen.