Storage limits. (Some people literally (literally "literally") never delete a message. My father-in-law is one.)
Privacy. ("I don't want my ISP reading my email." Yes, really.)
Ignorance, laziness or incompetence. (Sadly, not all technicians are created equal.)
It happens less when people use their phones as their primary email device, but it still happens.
1. Storage - Hence the reason I will not support clients, as in say, "Hell, no!," in a businesslike way, if they insist they can NEVER delete ANY message. Email is no different than real mail, and if people cannot understand that, and know that most email is "read and pitch" then I don't want to deal with their inevitable problems. In my opinion, and that's all it is, we do our clients a disservice if all we do is keep enabling the untenable. I've been using IMAP for decades, literally, and am not as good at practicing what I preach as I wish I were, but I'm still under 10 GB for email and will likely never, ever exceed that. Read and pitch; it's easy. What needs to be kept/filed is the exception, not the rule. Being an enabler is not a part of my personal job description.
2. Privacy. Any time anyone says something like what you quoted to me, I say, "Well, don't you think that horse is out of the barn. They have ALL your email messages before you ever see them, and if they wanted to read them (and they don't), you could not stop them." What they're saying is roughly equivalent to saying that you don't want the Post Office to handle your mail because they might read it. And if you believe that either your email provider or the postal service is reading your (e)mail, you're in tin foil hat territory.
3. Sadly, yes.
Part of our role is as educators. Always has been, always will be. We all set our own limits about what we will, and will not, do and put up with.