We don't know what Apple does with their data. They might print it out and make paper airplanes with it. They might use it to test their backups. And yes, they even might sell it. Or, they may do nothing with it at all. We just don't know.
I'm not accusing them of misusing our data, but I do know that they have, in the past, chosen to do something that looks like they are disregarding their user's privacy in order to make a buck (https://www.cnet.com/news/apple-moving-icloud-encryption-key...).
And they can set up their systems in a way that ensures they aren't violating our privacy or trust, but they choose not to do that. That to me, is very suspicious.
I do agree that they seem like the best tech giant for user privacy, but that is a super low bar. Just because they are "the best", doesn't mean they are good at it.