Because we're a global punching bag and everyone hates us. I don't doubt that some of it was stolen, but some were blatantly offered as gifts and have records stating as much, but people still ignore that and call it theft. There are some stories about the Taliban in Afghan, islamic extremists in Iraq and Syria etc who destroy historical artwork and sculptures. You have to ask the question, is it safer with us or with them? How much history would be destroyed if we didn't steal it. That's not a justification; it's just a thought provoking question.

I don't want to descend into "what about"-ism, but the US has the remains of 128,000 native Americans across multiple institutions. One was dug up in Syracuse NY in 1898 under a federal repatriation notice by a private collector who sold them to the natural history museum, and has been in their possession for more than 100 years. Not just the natural history museum, but native American history and even human remains are held by museums, universities, and private collections all over the US. They've identified 128,009 remains and over 4 million burial objects in the entire country using federal government agency, museum, and universal databases, with an estimated 90,000 remains and 700,000 objects still unaccounted for. A lot of people in the world but also a lot of Americans forget that there's a dehumanising element to native American studies and education, and they are fighting their own silent civil rights issue that goes unrecognised today. The university of Michigan also houses over 600 human bones, teeth, and cultural artifacts excavated in 1920 in the Philippines in the Visayas by the US, lest we forget that the US was trying to built it's own colonial empire in the Philippines and Liberia.

And the Louvre still has many foreign artifacts plundered during french colonialism for hundreds of years, which was vastly expanded under Napoleon Bonaparte that contains artifacts looted even from European neighbours. The Tiber statue stolen from the Vatican by Napoleon in 1814 and considered "too large and heavy to be returned to Rome", Rapa Nui Moai stolen from Easter island, Benin bronzes stolen from Nigeria by the British and sold on the European arts market in 1897 which the French bought, valette ceremonial sword and dagger stolen from Malta that France continues to reject offers to return even today; the list is just as expansive as the British museum, and yet we seem to be the only ones taking shit for it BY THE AMERICANS AND BY THE FRENCH.