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Because the internet is in a fad where it's very fashionable to require evidence for any claims, and most opinions.

Citation?

The title is stated as fact, not a theory. You have to get all the way to the last sentence in the abstract before they say it's a hypothesis.

Misleading title at best. I would say disingenuous.

Its clickbait, clickbait works, you see lots of discussion here thanks to it (or due to if you don't like it).

Because they stated a hypothesis. In order to be a theory, it needs to be rooted in data.

It's everyday English though, theory is the common word for what would be called an hypothesis in scientific circles.