Making gaming video's isn't only about money.
It's easy to make, easy to upload, the companies get free advertising through that, and other people enjoy it.
To take an example on this Forum:
When NSM couldn't decide which Mario Party version he had to pick up, I've made an hour long gameplay video of Mario Party 6, and thanks to this, he was sure enough to buy it.
On the other hand, it was a second hand game, so Nintendo didn't earn anything on it, but it was just to give an example.
Also, Nintendo sends us some Trailers through email as well, and it's up to us to get it on the internet, on TV, in the cinema's, in a Video Review, or any other place/stuff you can display video's.