Why change the name of Master pages arbitrarily? It's confusing and disturbing to ordinary users.

It wasn’t changed arbitrarily. Adobe changed “Master Pages” to “Parent Pages” in InDesign 17.0 in 2021 as part of a wider move towards more inclusive terminology. You can agree or disagree with that reasoning, but there was a stated reason for the change.
 

Nothing about the actual feature changed. A Parent Page does exactly what a Master Page did, and Adobe’s own documentation still describes it as “previously known as a Master Page”, precisely so existing users can make the connection.
 

I can understand saying, “I preferred Master Page because I used that terminology for 20 years.” That’s fair enough. But “confusing and disturbing to ordinary users” seems rather dramatic for a terminology change that happened nearly five years ago.
 

And “who in their right mind cares about it?” rather answers the question from only one perspective. Clearly some people did care about the terminology, which is why Adobe changed it. You don't personally have to care about the reason in order to understand that somebody else might.
 

If anything, the useful criticism would be whether Adobe handled the transition clearly enough for established users, rather than pretending the change was made randomly.