Why does it have the same name as the 1982 film when it's a prequel?

Just to mess with people, I guess.. Also, it seems to be a trendy thing to do in movies these days. There was a "Spider-Man" movie in 1977 and another "Spider-Man" movie in 2002 (same name). Also, there was Star Trek: The Motion Picture (1979) and then the JJ Abrams Star Trek in 2009 (it didn't have "The Motion Picture" in the name, but being simply Star Trek, I think its name is similar enough).

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Because it's a remake.

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You're right, but technically the 1982 film's title is "John Carpenter's The Thing"

Forever 9 Angels

Splinter would have been a good name.

They basically couldn't think of anything satisfactory. I think they considered "The Thing: Beginnings" but gave up.

Well, it's the same Thing after all.

It was very purposely named this way because it's a stealth prequel disguised as a remake. There are clues that it's a prequel, such as when Carter leaves the axe in the wall (the Americans find the axe in the original movie), but these clues are subtle enough that most audience members will completely miss them, as the original came out 29 years earlier and we're not likely to remember its details. The twist at the end, when Lars fires at the dog from the helicopter, is meant to jump out at us and make us realize all very suddenly that Oh my god, this is the beginning of the other The Thing. It isn't a remake at all!

Stealth movies are hard to disguise, so there are very few of them out there. [spoiler]Ones I can think of off the top of my head are Split (2016), which is a stealth sequel to Unbreakable (2000), and Mute (2018), which is a stealth sequel to Moon (2009).[/spoiler]