As Arthur C. Clarke wrote, any sufficiently advanced technology is indistinguishable from magic. In the case of bioprinted fairy drones, the tech only looks like magic because it isn’t advanced enough.
Bioprinting is the 3D printing of organic material. It’s been demonstrated for years in various different capacities, but the current state-of-the-art suggests that we’re as far from printing a fully-functional organ as a we are from inorganic 3D printers printing a fully-functional car — you can do something that superficially looks right, but doesn’t have all (or even a bare minimum) of the functionality.
Some of the problems bioprinting has are even the same problems that inorganic 3D printing has: There are a lot of different cell (/material) types, and you can’t get away with using the wrong thing. Just as jet engines don’t work too well when 3D printed out of pure plastic, you don’t want to mix up kidney cell types (plural: there are multiple types) with artery cell types.
Other problems are unique to bioprinting: while houses and boats (or rather, the empty shells of houses and boats) are limited only by the range of the printer, organic material has a tendency to die very quickly if it doesn’t get any oxygen, and getting oxygen into tissue without a heart is very difficult. Difficult, but for small things, possible, and that’s where fairies come in.
Fairies, at least in their Victorian-era depictions, are tiny. Not actually small enough to deal with all the oxygen diffusion issues by themselves, but small enough that it’s plausible tissue could be printed in a cryo-preserved state (which does work, just not for human-sized creatures), and then the complete organism thawed out alive when printing is finished. Their diminutive size also makes their wings actually plausible, whereas a human-sized biodrone would need ridiculous wings to fly.
At this point, normal people will be asking ethical questions about their brains and lifespan. As they’ve been printed, this is absolutely the wrong question: you absolutely should not even try to print a brain into them in the first place — and not just because of the ethical dimension! We couldn’t even design a functional brain yet because we don’t actually understand brains very well (if we did, every A.I. question from self-driving cars to social media moderation would already be solved), but even if we understood brains perfectly, the brain and nerve tissues are particularly awkward one to print as axons and dendrites give them pointy bits which go all over the place in ways which directly matter to them being useful.
So, instead of giving them brains, give them WiFi. Instead of eyes, give them cameras. Congratulations, you now have a bioprinted fairy drone.
You may ask: Why?
Fair question. Other than size-fetishists, who benefits from a tiny flying humanoid robot? Well, pretty much everyone. While they couldn’t do any heavy lifting, the entire history of human invention all the way back to the inclined plane, the wheel, and fire, has been to minimise our heavy lifting. What tiny flying human-shaped organic robots can do is not limited to themselves, but part of the entire ecosystem of machines in our world, one of which is swarm robotics that lets them work together much more effectively than a mere team of humans, and at basically the same range of tasks.
So, my answer to “why” is a slight variant on an old meme of a question: Would you rather compete against a single 1.8m tall human, or a thousand pocket-sized fairies all working together?