Humans are space dragons (aka "human resources")

Selling and buying parts or byproducts of sapient species is heavily banned, totally immoral, and judged in any part of the known inhabited space. Unless it's human parts.

During their space exploration age, far before meeting an actual sapient neighbor, humans started multiple medical programs aimed at increasing their naturally high adaptability to the point where it became an existential imperative. Where other species would find an organically toxic world with deadly flora and fauna, humans would see such a world as the perfect candidate for life. And where other races would find it cheaper to settle a dead world, humans would conquer living ones. No matter how illogical it would seem. An important part of such efforts was fitting into the ecological niche on the new world.

The main engine of such an approach became medical research, new food sources, and human greed. And when humans actually got access to the galactic database, they took great interest in the list of deathworlds that other species refused to even set foot on beyond rare scientific missions. The galaxy knows this period as the "Expansion of Madness," when humans poured onto deathworlds like madmen, killing themselves but rushing to explore and find new lifeforms to farm. Humans know this period as the "Spite and Spice War", when human corporations, adventurers, scientists, and governments would pay huge money just to get access to extraterrestrial extreme lifeforms. To use as alternative chemical production techniques, to get new drugs and stimulants, to grow food products that others would consider poisonous—yet for humans, these were the possibilities to survive under any condition, as growing a parasitic fungus with hyperdimensional replication mechanisms is cheaper and tastier than eating reprocessed organic waste, like most space-exploring species do. Humans fought each other with natural and chemically induced devotion for those "treasures" of space that other species might as well use as highly cruel bioweapons.

And that's where it might have ended. Until someone found out that in their madness, humans had turned their species into a cauldron of knowledge and practice, and became themselves something... really profitable.

Eating human food will end you in a very cruel way. Yet human gastric juices and microflora are the most accessible and efficient chemical neutralizer there is. Being sent to a human hospital seems like a less harsh penalty than a life sentence in pain simulation. Yet human blood will quickly heal you, and even a small amount of it in your system will greatly boost your lifespan expectancy. The human brain might be an image of hell for any psychic race, but humans adapted to consume extradimensional organisms and regularly use "Screaming Souls of the Damned"—the marketing name "Blinking Coffee (TM)"—as a daily drug, and their neural chemicals (if that word still applies) are a guaranteed way to gain psychic powers for any alien race. (It's surprising how humans don't naturally have them.)

And so while humans actively hunted new extreme lifeforms to use, many aliens started hunting humans. This caused a large number of retribution wars, turned human kindergartens into the second most guarded objects in known space (after the mythical Overlord's Palace), and caused several "Wars of the Damned," where desperate aliens—deadly ill or completely broke—attacked humans with desperation in hopes of being saved from death or gaining riches to fix their economic conditions. Even humans' allies, who refused to fight humans for harvest, were often found buying and selling human metabolic byproducts and even trafficking passed ones with different levels of legality. (Guarding this market and friendship with humans became a reason why their allied fleets joined the human ones, together making them the sturdiest line of defense in the universe.)

Eventually, human expansion slowed down and humans themselves became far less trusting. It's hard to fully trust anybody when most of the galaxy sees you as a walking money bag. And even threats to their whole species won't save humans from those who might actually try. The myth of humans who live in the most inhospitable places, surrounded by guardians who became rich and strong just from living near humans, who nest in riches and wield war technologies of an unknown level, boosted by their need for self-protection—this myth spread among the galaxy. Spacesuits made of human skin became artifacts that passed through generations. And lords who got access to potions made of human blood were known as immortals. And nowadays, a human colony settling on a deathworld in your space is seen as both curse and opportunity. Their fleet, their technologies, and their allies will leave you very little chance. But even those slim chances are enough to make many try and rush to their heavily protected lairs, in hopes of at least seeing a fraction of their riches. And obtaining at least a small fragment of their bodies.