If Windows 8 is already at RTM, why aren't there any commercials yet?

It is a way of continuing to make a profit. They do not want the consumers to wait for something and thus loosing money and profit.

Microsoft still wants to get people to purchase the operating system available and later encourage them to pay more and upgrade

There are a couple of commercials that mention it, but they usually only mention that "if you buy a PC today, you can upgrade to Windows 8 for just $15!"

Microsoft, along with PC Manufacturers, won't cannibalize their current sales buy advertising a product that isn't ready to ship just yet. When it comes to electronics, you don't want to advertise then newest thing until you get rid of most of the older stuff first. Apple won't even mention their newest products until a week or two before shipping at most (unless it's a developer announcement).

keep in mind these advwetisments you are seing now on TV for back to school and all were all made months months ago even before school was over in some cases. at that time there was no set date for win8 release and there is still no promise it will be released they could still find something and hold back most sales of this OS will be online down load and at time of purchase with new devices ..