The people still choose reddit/Facebook/google. I don't know we're supposed to change that without actually removing people's freedom of choice.
In my opinion/experience, it's for a few reasons. People are marketed these centralized platforms, typically they're very/fairly simple to use, and those platforms already have an established userbase. Combined with the other factors, the userbase will keep growing, which also incentivizes Even more users to adopt the platform.
For most people, there's no incentive to use some small random forum. And these small random forums aren't typically run for profit, meaning people aren't paying for ads for their niche forum or hobby website because it's just a hobby, not a business run for profit. Whereas people will see countless ads for Instagram or TikTok. Typically, people who don't block ads, and use these sorts of media didn't care enough to bother looking for alternative platforms, they couldn't even be bothered to set up an adblocker.