This is how all the child metas are set up (with the exception of meta.stackexchange.com). There's no 'setting' to enable it, so it would need a systemic change to make that happen.
I checked into why it was set up this way, and best I can gather is it's because the child meta posts are generally more geared towards people who are actively using the site. That makes it awkward to share this stuff broadly across the broader Internet, especially given that new visitors can't even meaningfully interact with a child meta.
But I don't particularly agree with the distinction —
We shouldn't assume that someone's audience, motivation, or need for sharing is wrong for this site. Bigger picture, folks become accustomed to how these tools should work, and when they don't work the way you'd expect, it makes for an unhappy, head-scratching experience. This isn't exactly a game-changing problem, but adding such exceptions in behavior between meta and the main Q&A should have a much greater impetus before justifying the inconsistency. I'm going to leave this a feature-request.