It’s just a hoax and prank. Some do it for the ad money. Some of the Nibiru channels there are earning their owners thousands of dollars a month of ad revenue. Others are videos by members of the public who think that if they see any bright light in the sky that they don’t understand, in any circumstances, then that means it is a planet called “Nibiru”. If they had somehow been persuaded that any bright light is Santa Claus and his reindeer we’d have lots of videos uploaded with sightings of Santa. They don’t see a planet, they just see unexplained lights.

It’s not true that astronomers are murdered for speaking up about “Nibiru” as another answer here says. This rumour derives from two sources

• Robert Harrington’s theory about a planet beyond Neptune was disproved just six months before he died of throat cancer. The conspiracy theorists reverse the order of those events and say he was killed to stop him talking about it.
• A postgraduate student prank video about the mysterious “professor Kaplan” - actually a postgraduate student bored in an idle moment in an observatory

Robert Harrington, photograph from his obituary Bob Harrington Obituary - he was not murdered. He died of throat cancer six months AFTER his theory of an extra planet beyond Neptune was disproved by Myles Standish using data from the Voyager 2 flyby of Neptune.

See also

• Debunked: Astronomers get murdered by the secret agencies if they “speak out” about Nibiru
• Debunked: NASA has been tracking “Planet X” for decades
• Nibiru Bullshit Tester - How to check if they know anything about astronomy
• Debunked: Nibiru will hit Earth on [Insert Date here]
• List of the articles in my Debunking Doomsday blog to date.
• Why the idea of Nibiru seems non refutable - and how does astronomy really work?

On wormwood see Debunked: Nibiru is Wormwood so must be real