Title case makes things stand out
Title case should be used when you're trying to make something stand out and grab people's attention, such as in a bold newspaper headline. As we've already got formatting to distinguish the titles from the content (bigness, position at the top of the page) we don't need to put them in title case to make them stand out.
In fact, most people reading the site will already be on the page; making things grab their attention will merely make it harder for them to read the rest of the content without their eyes jumping back to the title every so often.
Question Titles Are Used for Other Things
As You Can See, It Is Difficult to Scan Titles.
It Is Equally Difficult to Read Them as Your Eyes Are Drawn to Words That Aren't Relevant.
Newspaper Headlines Have to Be Written Carefully to Take Advantage of This Effect, and so Draw Your Eyes towards the Meaningful Content – Headlines Are Shorter than Most of Our Question Titles as a Result.
Given That Titles Are Used in Search Results, the Linked and Related Sidebars and Also in the Hot Network Questions List, in Addition to Anywhere the Question Titles Are Used, It Is Preferable That They Are Readable.
Most questions on this site are written in sentence case; consistency trumps most other things. This answer aside, most of the headings I've seen are written in sentence case.
Every single resource I can find on title case (including the link provided in the question) says something like this:
[Sentence case], recommended by the Publication Manual of the American Psychological Association for titles in reference lists, is popular with many online and print publications. In fact, it's now the standard form for titles and headlines in most countries—but not (yet) in the United States. — Richard Nordquist, via ThoughtCo.
Not only is this our site's standard, and the world's standard, just look at the Hot Network Questions sidebar; it's the Network standard.