While form rejection letters aren't always a thrill to receive, they are sent most often in the voluminous correspondence shuttled through the mail (or email) between writers and editors. Much of this boils down to numbers. If a publication receives hundreds of submissions a month, they would have to designate staff whose only task was to write up all those rejections. That doesn't include the time it takes for the editor to articulate thoughts about why the work was rejected. Personalizing every response would prevent editors from doing what they're supposed to be doing: putting together a stellar publication of creative work.
In many instances, the form rejection says just what the editor wants to say: your submission just wasn't right for the publication. Detailing why that's the case may not always be particularly helpful to you as a writer, either.
Learn what you can from any rejection you receive, particularly if an editor has taken the time to write a note about your work. (It does happen!) Use it as an opportunity to see your work through that particular editor's eye. But keep in mind there are other outlets for feedback to develop your skills, such as creative writing classes, workshops, and writerly friends. And keep reading the journals you want to publish in; it's the only way to assess whether your next story should be submitted there.