It’s because they don’t use the same wireless networks as phones and have like a 96h battery life. That’s why hospitals give them to doctors. And plenty of people still use walkie-talkies because they work (within a certain range) even when nothing else does.

Also, the pager companies are still in business mostly because of the pucks you get at a restaurant that buzz when your table or food is ready. “Obsolete” tech that’s dirt cheap can sometimes hang around for niche use cases.