Why do people still believe the idea of keyword density?
Personally I blame Web Position. Many years ago, when their product was a desktop product called WebPosition Gold, they used to produce reports based on individual pages, containing nonsensical figures:
“A word count from 123 to 467 is suggested for the Body Text area. Your word count is 939 right now so you might consider decreasing your word count in the Body Text. A keyword prominence of at least 23% is suggested for the Body Text area. Your prominence is 4.6% right now so you might consider increasing your prominence in the Body Text.“
To this day there are still people who believe that you need X words in the title, Y keywords in the meta tag and a keyword density of Z%.
There are also people who genuinely believe the world to be flat.
Both ideas are absolute nonsense, based on out-dated hokum.
Google do a good job of finding the content on your pages. You can certainly help them, and if you know what you’re doing you can tweak your chances a little.
But applying meaningless figures onto out-dated silly ideas is pointless.