That question doesn't really make sense in the context of a US workplace.

The employee:HR relationship is fundamentally adversarial over here. HR's job is to protect the company, including from its own employees. They will throw you under the bus if it benefits the company.

Most people won't run into issues with them, but if you think that someone important to the company's bottom line is behaving inappropriately and you go to HR about it, how would you expect them to react? It's a matter of incentives.

Why would you trust any employer’s HR department? Their job is to reduce the company’s liability, not help you with your problems.

I live in Mexico and it's certainly not because I trust the police.

HR exists to protect the company not the individual

Because you don't truly find out how bad/toxic departments within a company are until you've been there a while.

Because "do I trust HR?" (a question to which the answer is almost certainly always "no, but it doesn't matter in most cases"), is only one minor criteria amongst a vast plethora of other important ones when deciding to join a company or not?

If you do, why were you offered a position?

What kind of a question is that? The first and foremost function of HR is to protect the company, you have no reason to trust them. And who even has such a deep insight into the HR of a company at the time of your hiring that they decide whether to take the job or not based on that info?