Turn it around. Imagine that you have several offers from different companies.
After you choose one (and you can choose only one), do the other companies have reason to get all mopey and feel offended to hear no reason for the rejection?
Of course they don't. Why should candidates feel any differently when a company chooses someone else?
How does a candidate improve on himself/herself if a company gives a stock phrase? Maybe doing a mock interview, but it's difficult to replicate the stressors associated with an interview.
After you choose one (and you can choose only one), do the other companies have reason to get all mopey and feel offended to hear no reason for the rejection?
Of course they don't. Why should candidates feel any differently when a company chooses someone else?