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The only reason we think this is so we can justify doing horrific things to animals. Most of these theories, at least in the West, originated from the Christian doctrine that humans are created in God's image and animals are just mindless NPCs. I've noticed that tons of people in the West, even if they're atheists, have still internalized this part due to how pervasive it is in Western culture and think humans are intrinsically better than animals just because.
Bonus boiling hot take: how scientists treat lab animals are some of the worst, most inhuman bullshit, so it's no wonder it took them this long to acknowledge they have these traits. Going through a science degree at a university (albeit not one that directly involves animal testing I just read a lot of papers on it, I studied animals in their natural habitat) has given me a respect for every part of science except the people who do animal testing. It's not even that the animal testing is taking place, I know it's necessary, but it's their indifference to the animals they're testing, absolutely zero effort made to reduce their suffering even by a little. And a lot of them will straight up tell you that reducing their suffering is more expensive which is why they don't bother, or they'll even try to spin it like "we have limited funding so by treating the animals like shit we can get more experiments done" like that's an excuse. They're so desensitized to it that they see the absolute hell they put them through as just part of the scientific process. It actually reminded me of the Auschwitz and Unit 731 experiments just not with humans, and that's supposed to make it okay apparently. IMO it's always dangerous when scientists start seeing their science as nothing but abstract concepts and stop caring about the real effects their experiments have in real life.
Also, a lot of the time it's not actually "necessary." Vaccine and pharmaceutical testing, I still don't like how they're doing it but I'm not about to say anyone should boycott vaccines and medicine. But the anti-aging shit being tested on monkeys which they'll promptly kill in a gas chamber afterward gets my blood boiling. You don't get to find ways for humans to live forever by torturing and killing monkeys, that's comic book villain shit. And the efforts to find viable alternatives to phase out animal testing is going at a glacial pace mainly because no one in charge of funding science really gives a shit about how many animals are tortured and killed, and from a business perspective, the lab animal suppliers feel threatened by any efforts to move away from their disgusting "products." And that's fucking shameful and negatively affects all of science.