A lot of people think that if a keyword is not in the title, url, and meta description, it is easy to rank.
But it always doesn't work the same way.
They don't know Rankbrain is working behind the scene.
One concept may be explained with different keywords.
For example, FREE PHOTOS DOWNLOAD, FREE IMAGES DOWNLOAD rank almost the same sites in SERP.
Because they talk about one concept, but people search for different keywords.
Why?
Because every person searches differently based on his vocabulary and knowledge.
Below is the screenshot, here the keyword ON PAGE SEO ranks for more than 1000 different keywords with different positions in SERP.
That's why, Moz Keyword Explorer and Ahref Keyword Explorer don't take ON-Page factors into consideration in order to check the weakness of a keyword.
You need to use a little bit more brain in order to check :)
I don't say that you should not look for On-Page factor, you should look for it. You need to check with different keywords that have searches and open the articles to check deeply.