I don't know why this is not implemented till now but this is very important for any post to be flagged. Even if we close a question then also there is a reason for that, but if a user is flagging a question, answer or comment then he is not giving any reason. The author of the content should know the reason for that flag. This way he can actually improve the content and community can be improved.
This also happens in Stackoverflow and should also be in Q2A as Flagging is an important feature.\
How does flagging happens in StackOverflow?
I've had similar thoughts too. I think it may make sense to have the same UI for flagging as for closing questions. As mentioned here I'm gonna look at improving closing so I'll work on flagging at the same time.
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April 20, 2021: Q2A 1.8.6 released!