multiple reasons :

• I have some friends I want to share with that might not want to share their email address with my family and vice versa.
• There is maybe a limited number of family members that know my personal email account and I want to keep it that way.
• I have multiple emails depending what app I use and do not want to share them.
• just not wanting to share an email address should be reason enough wtf
• Using authentik, I might not want people to know what email I use for other apps.
• I have a kid in my users and their info is all of a sudden public, even for a day? huh-uh, that's totally legit.
• I don't want to share my private email to other peoples guests.

There are so many usecase that make this very not a normal configuration. So many apps exist that let you hide your email, this is common sense.

I can remove the email from nextcloud, but if the person wants to get notifications : gotta chose between what is more important.

I also know the majority of my users don't care enough about it to know that the information exists, but then they share to someone that does and you just game your whole families personnal information to someone on a silver platter (hopefully they are a nice someone).

ps: this isnt only about the email, it is the lack of care for private information in general from something as big has Nextcloud. There are so many ways I can get my (and others) information stolen as it is, I will not use an app that just makes it normal and easier then it already is.