We know that Meta tracks everything you do in a pretty invasive manner. We know they use this data to target ads, and while they claim not to share data with advertisers, we do know that people have figured out ways to leak some of that data.
Since they share data with a lot of 3rd party tracking companies, we would have to trust all of them to keep our data safe. It is highly likely that some of these companies employ malicious actors.
Now, maybe you think you have nothing to hide. But the political landscape changes all the time, and things that were legal one day might be criminalised the next day, or some terrorist organisation gets a hand on your data and figures that you are an enemy of their god for some reason, ...
The only way to protect against these things is to not store the data in the first place.