Partly because it's such low cost to produce.

A single episode of television requires hundreds of people working for a couple weeks minimum and costs from hundreds of thousands of dollars to millions.

How much does effort does it take to write and record a song? It's closer to a handful of people. And it's usually measured in thousands of dollars, not hundreds of thousands or millions.

A few hit artists spend more because they can afford to, but even that still doesn't compare at all with hit TV/movies.