Very few editors or agents personalise all of the rejections they send out: most rejections are made by a form letter, with no detail or comment.
When work shows particular promise but still isn't quite right, then sometimes an agent or editor concerned will write a little note explaining exactly why it has been rejected. Perhaps it's too similar to another book that’s just been signed; perhaps it's an unfamiliar genre; or perhaps it's not quite good enough for publication yet, and needs further work to get it there. But why don't agents and editors do this for every rejection? A couple of lines don't take up too much time; surely it's not too much to ask? (If you think that last is true, then read Emma Darwin's comments here for some new insight on the practicalities involved.)
Editors and agents earn their living from the books that are published, not from the books that they reject; and the ever-replenishing vastness of the slush pile means that if they did personalise every rejection they sent they’d never get anything else done and would be out of business before the month was up.
Assuming, however, that our agent or editor has time available for these things, could anything they have to say be useful to those rejected writers? The majority of the books in the slush pile are so bad that it would be impossible to know quite where to start in order to explain why they were rejected. If an agent were to write, "I've rejected your book because it is dreadful in every way," would that be of any more help to the writer on the receiving end than a form rejection? The writers who are likely to receive such rejections are also unlikely to believe them: what good would it do?
Some writers see any personalisation as an invitation to develop a deeper relationship with the person who rejected their work: they’ll argue that their work has been misunderstood, or that more of it needs to be read in order to fully appreciate its many finer points. Or they might write back just to insult the person who rejected the work (I wonder what writers think will be gained by this behaviour: do they hope to change someone’s mind just by insulting them?). But there’s a more worrying side to this, and it’s one I have first-had experience of. A writer I once sent a personalised rejection to ended up stalking me. He sent me more and more submissions; phoned me every hour; and ended up sending in photos of me arriving at work in the mornings, going out for lunch, and taking the bus home. Now, that was scary stuff and I know of a couple of other editors have faced this too. Like me, they switched to form rejections for just about everything as a result.