Why do black holes have such strong gravity?

At the end of one of my favorite movies of all time, The War of the Worlds (the 1953 version, the only version, there was no remake, I can't hear you, lalalalalalalala), the narrator says that it's the littlest things that are important, alluding to the microbes that (spoiler alert) wipe out the aliens. I always hated that voice-over at the end, actually. But he's right. In this case, big things might be stars, and little things are black holes. OK, so what does this have to do with anything? Well, in my live video chat, Cate Mato asked, "Why is the gravity from a black hole so much stronger than the star from which it forms?" That's a good question. I have a good answer. Maybe you've already figured it out, but just in case, here's my response. And I get to plug my book! Bonus.