The "hide" link is right next to the "flag" link. Using flag instead of hide puts more strain on the mods, and is not the right thing to do for "this topic doesn't apply to my interests."
But it is the right thing to do for "this topic violates HN guidelines both in letter and in spirit, as well as predictably causing low-quality discussion threads".
What if I would prefer that these topics don't show up at all?
What if I'm concerned that leaving such topics up would attract more of the kind of people that prefer discussing these topics over tech topics?
Hiding doesn't fix the problem.