Because Google optimizes for "quality content" since at least the Penguin update. They're using NLP tools to assess the writing quality (similar to algorithms telling you at what school grade level your writing is).
This was good, because it cured all the copy&pastable 2000s era "tag cloud" sites which simply dumped tons of search keywords all over the place.
Ideally, it lead to a stronger emphasis on high-quality human-written content, but it turns out that this algorithm, again, is easily fooled by feeding it "SEO essays" that looks like prose, but is irrelevant text gibberish, but written coherently.
That lead content creators to expand data that would ideally be presented in tabular form on one page, to multi-page "SEO prose" that looks like it's written for humans, but is completely undigestible.
That, along with Google's auto-suggestion feature that finish your sentences after you type in some words, especially on mobile, lead to the impression that people actually like to search in full sentences.
This was good, because it cured all the copy&pastable 2000s era "tag cloud" sites which simply dumped tons of search keywords all over the place.
Ideally, it lead to a stronger emphasis on high-quality human-written content, but it turns out that this algorithm, again, is easily fooled by feeding it "SEO essays" that looks like prose, but is irrelevant text gibberish, but written coherently.
That lead content creators to expand data that would ideally be presented in tabular form on one page, to multi-page "SEO prose" that looks like it's written for humans, but is completely undigestible.
That, along with Google's auto-suggestion feature that finish your sentences after you type in some words, especially on mobile, lead to the impression that people actually like to search in full sentences.