https://www.realclearpolling.com/betting-odds/2024/president
Because enough people wanted to bet on it and they make money on every bet.
The electors shall meet in their respective states, and vote by ballot for two persons, of whom one at least shall not be an inhabitant of the same state with themselves.
<https://www.law.cornell.edu/constitution-conan/article-2/sec...>
In strict compliance, this would require California's electors to split their votes on candidates from other states, though other states wouldn't face this limitation.
History.com addressed this earlier this year: "Can the President and Vice President Be From the Same State?" <https://www.history.com/news/can-the-president-and-vice-pres...>
So no, a Harris-Newsom ticket is highly unlikely.