why do people with private pages ( Facebook, Instagram, ETC) block others from adding them?

[Sara]

Maybe because some of those accounts are exclusively for family and / or private matters, like a diary/album?
There's always a bazillion people asking to be your friends in FB, whether you know them or not, so maybe they don't want to go through the hassle of manually saying "no" to every single one of them.

Sometimes even if you say no, some people just don't know when to quit and keep on trying, so blocking is the best way of telling them to "bug off" for good. Its a lot easier to manually pick the people you WANT to be your friends.