alt Hacker NewsI suspect two reasons. First, it's not easy to make your own blog - what are you thinking of here - self hosted wordpress, which will surely get hacked? If you use a service like WPEngine, that's no better than using substack.
Second, Google only surfaces the big platforms any more.
I'm talking about developers specifically. I understand if non-tech people would rather delegate all of it to a third-party like substack or medium, but as a dev, a blog is about the next hard thing you could make after a to-do app, especially post-AI.
As for your second point you're probably right, it depends on what your intention is with your blog. I wouldn't mind having thousands of people read mine, but I personally think it's eventual as long as you're a good writer and hang out in relevant communities, but I guess it makes sense from an SEO pov.