Because they literally had to change the definition of vaccine once people pointed out that this didn't prevent you from getting covid.

Don't worry, though. The fact checkers tell you they did this 'for accuracy'. In doing so, of course, they redefined sunlight as a 'vaccine'.

https://www.usatoday.com/story/news/factcheck/2021/11/30/fact-check-merriam-webster-changed-vaccine-definition-accuracy/6354415001/

According to an archived version of the dictionary's website, Merriam-Webster formerly said a "vaccine" was "a preparation of killed microorganisms, living attenuated organisms, or living fully virulent organisms that is administered to produce or artificially increase immunity to a particular disease."

The new definition of "vaccine", published in May, reads: "a preparation that is administered – as by injection – to stimulate the body's immune response against a specific infectious agent or disease."

The editors changed "artificially increase immunity" to "stimulate the body’s immune response" because they believed it would be more helpful to readers, according to Sokolowski.