Because the title of the post should match the title of the submission - unless the mods dislike it, then it'll be changed to something else.

The mods dislike many titles but only change some of them. We also like some of the ones we change!

That's because there's a principle involved. It's not hard to find: see https://news.ycombinator.com/newsguidelines.html, or https://news.ycombinator.com/item?id=46671377 nearby.