The main reason we don't let users change their own email address or phone number is to hinder scammers who want to create a duplicate account, who otherwise might change their email address on their old account so you can re-use on a new account.
Secondarily, we don't want people purposely changing their email or phone number to a non-working one (or someone else's) as a sloppy way to unsubscribe.
For postal address changes, we would also need to require the user upload a new ID and re-do any postal address verification steps.
IMPORTANT: If user is changing their country (in their address) or changing their country code for the phone number, this needs to be held and require manual admin approval. That is because it is much rarer that a reviewer will move to a whole new country, and instead could be a sign they lied about their location.
March 27th Update: Users can now change their phone number on the Payment Profile page: