Why can't I see how many helpful flags a user has, in his/her profile? Is it considered as private information? It's something similar like review which I can see at least when hovering over an image in recent reviews.
Not sure on the official reason, but being able to see others' helpful flags could influence some users in their opinion of a user.
Affecting their decision towards answering a user's question or not, or influencing how they vote up/down that user's answer/question.
Which is unfair, as the community moderating aspect of the site is voluntary, and a user shouldn't be judged just because they've not had any helpful flags. That user might not help out at all with suggested edits or reviewing etc, but their answers and/or questions are in themselves providing invaluable content to the site.
Also, I don't see any need for being able to see such information on other users, other out of interest/being nosey etc
Yes, flags were originally designed for diamond moderators in mind, and information about these flags is strictly confidential.
Today's flags include a variety that only appears in the review queues and is not seen by us in the moderator queue, but at the end of the day flags are still a fundamental moderation tool, and therefore the only people who should be concerned with a user's flagging information and statistics are moderators and members of the SE team. So, as James has mentioned, there is no need for the average user to be able to see this sort of information; curiosity/nosiness alone is not a good reason for the system to reveal this sort of information to just any user.
To reinforce the privacy aspect, note that users with at least 10k reputation are considered to have "access to moderator tools", but for the sake of confidentiality all flags listed in the 10k queue (i.e. that are not limited to diamond moderators) are anonymous — you would only be able to see posts that were flagged, but not the users flagging them. That is something only we can see, and are under no obligation to disclose unless the user chooses to do so on their own. (In fact, moderators have to abide by the moderator agreement, which states that we are not to disclose information that only we can see.)