Truth is a complete defense to defamation in the US. The problem is SLAPP protections don't go far enough when there is a large disparity in resources.

It's especially problematic when the consequences to victims go beyond lawsuits (reputational, mental, and physical well-being are all potential threats.)

The problem isn't that they would lose, it's that it is expensive to litigate. Hiring a lawyer and fighting someone who will keep appealing no matter what gets very expensive very quickly. Not to mention, if it is someone with clout, they can drag your reputation through the mud while they're doing it.

Freedom of speech exist

But the problem is Freedom after speech. 👀

You can't lose, really. But they can drown you in lawyer fees. Straight up, the letter of the law in usa we have some of the highest burdens of proof for showing defamation. You have to prove that not only the person knows they're lying about something, but also that significant damage was done. You'll find European countries have much lower bars.