Because it's not always about their entertainment. I know churches that post info about events only on WhatsApp groups, if you don't use it - you're screwed. I know kindergardens which use Facebook Messenger groups to send announcements to their parents' children - if you don't use it, you will miss important info.
For most people, letting go such things is very impractical. One can try to persuade for a better way to do something - but then you become the problem.