They are partisan. It's just whoever decides to make the bets. Like the 2022 election situation was just one big whale. There's just nothing about it that makes it inherently non-partisan.
A partisan bet makes the opposite position slightly more profitable, attracting bets which equilibrate the market back at fair prices (i.e. realistic probabilities)
A whale just means the market gets more mis-priced, creating greater incentive for correction, which typically occurs.
Let's for a moment say you're right, and prediction markets do have a flaw (that if someone wants to spend a lot of money making an outcome look more/less likely), that's an imperfection vs the many biases in other information sources.
tl;dr prediction markets financially penalise providing misleading information, whereas traditional media profit from misleading information