The first reason why this doesn't make any real sense is that it's not easy to write a page about just one keyword phrase with a natural flow
. In addition, having a page devoted to only one keyword when you could have used several is simply a poor use of space on your site. What is the point of targeting a single keyword when the same page could be optimizing several keyword phrases? The more keyword phrases you have on your optimized page, the better it is. You'll get more targeted traffic this way. You can find out for yourself by looking at your web analytics that you are getting traffic from several, possibly many different keywords. You will have more success with the search engines if you target a variety of keywords on each page.
The next myth is about the content that you put on your site's page. People think that they need a minimum number of words on their site to help boost their rank. Content should be evaluated by quality of information not word count. The desire to use keywords more effectively gave rise to the idea that your site would need a minimum number of words. But that doesn't mean having a shorter copy doesn't work well.
A short copy can be used as effectively as a longer version. Search engines are needed to help drive traffic to your site but that site needs to geared towards your niche not just the search engines. Any amount of content is good - use your common sense to make it the optimal size.
You have to realize that it takes some time and study to make search engine optimization pay off for you. If you find yourself not knowing what action to take, refer to a site or forum you trust and seek an answer that makes sense. You don't have to be perfect, as everyone makes mistakes, but you can become skilled enough to get highly ranked for your sites.