I can only hope these people are simply good-natured, raised properly, that they willingly trust and believe in the inner goodness of every man. I don't want to think they're just eager yet naïve individuals who lack basic comprehension.
I know this is very common here in my country. If somebody wants to save an information, he/she sends it to himself/herself. Many DM their own accounts with important information they want to keep or they don't want to forget that they need every once in a while. Accessibility and convenience are probably the reason for doing so.
Many also don't know that gadgets could also automatically store their files in the cloud. The moment they're connected to a WiFi, their files are automatically backed up somewhere which is accessible online 24/7. Some might think keeping a photo of their seed phrase in a locked album is enough security. If they don't tinker with their settings, what's supposed to be kept offline is kept online 24/7.