I guess the obvious answer is no one finds it profitable to use the Google search network for those ads/keywords. But is that really it? I find this unusual. It seems to me you could easily get top spot in Google SERPs for some very high traffic keywords.
Sorry.. I admit that I'm a total newb to Adwords although I have read a number of ebooks and watched a few videos.
I have a client that is getting ready to launch a campaign in a country that is not only not familiar with their brand but the product is a brand new concept for that country as well. Our brand keywords that convert so well in the US are obviously not going to work for this new International campaign. We will have to use high traffic, general keywords to get brand awareness and to get their product in front of the right audience.
Consider putting your high traffic keywords into their own campaign where you can set daily budgets and still bid competitively. That way you can decide if you want to have those keywords show 100% of the time or show more on a test basis where your ads show off and on all day. If you don't put them in their own campaign, you risk running out of budget for your long tail keywords and/or your brand keywords that will have higher conversion rates and lower conversion costs.
I suppose it's possible that there are some keywords so generic that it's virtually impossible to advertise on them and keep a decent QS, but it seems unlikely.
I'm pretty sure the main reason you don't see the ads for some high traffic keywords like "myspace layouts" is that the CTR on ads ends being so low (well under 1%) that the QS becomes Poor and the minimum bid skyrockets. Nobody's going to make a profit on myspace layouts by paying $0.50 or $1.00 per click, and consequently nobody bothers to advertise for the term.
There's tons of ads for the keyword 'myspace layouts'. It's just that those ads don't appear in the search network. They only appear in the content network. Is it possible that the CTR on ads for 'myspace layouts' is a lot lower on the search network than on the content network? I thought the search network typically outperformed the content network.