Because Trump told them to release "any and all other information".
"In a post on his social media platform Thursday night, Trump said he was directing government agencies to release files related “to alien and extraterrestrial life, unidentified aerial phenomena (UAP), and unidentified flying objects (UFOs), and any and all other information connected to these highly complex, but extremely interesting and important, matters.”"
Trump says he doesn’t know if aliens are real but directs government to release files on UFOs, more
The fact that PDJT says he does not know is the first clue that they very well might exist.If he had absolute knowledge that they did not exist he wouldn’t say, “I don’t know”. So that raises the very possibility that they might very well exist. Sounds like he may be receiving conflicting information. They(Those pesky little guys in white coats tucked away in secret labs) do not always tell the president everything they know.