Answer:
On top of other answers about residuals, streaming services have been notorious about hiding their viewership numbers. This means that not only are actors seeing less from residuals, but it makes it increasingly impossible for them to figure out what their residuals could or should be. The lack of transparency is making it easier to screw over everyone at every level of production.
EDIT: Also, the offer the SAG just received included a line about actors taking 1 day's worth of pay in exchange for a capture of their likeness that could be used by AI forever-after,
without compensation.
On top of that there's an issue of solidarity. The ongoing treatment of the writers strike has made actors less likely to believe the studios will be reasonable or accommodating to their demands, or to take promises to resolve these issues as a matter of trust. There simply is no trust right now.
> I thought they were already some of the best payed in the industry.
The vast majority of actors aren't paid well, because they're working in the hopes of becoming a big name actor or for the love of the work. This makes them, as a group, exploitable and vulnerable. This is why people like Harvey Weinstein thrive in this environment. This is why union action is so important.