I also get form rejections on fulls. It's very disappointing. When I get a full request, I hope that even if they reject it they'll give me some personalized feedback. Sometimes I can guess they didn't actually read the whole book -- like if they reject the full 24 hours later. But other times they would have had to time to read it and still send a form.
In a way, though, whether it's personalized or not, it's still a rejection. If it was something they think would be an easy fix, they would request an R&R. Also, what one agent likes about your book another doesn't like. So if one agent says they didn't like your book because of X, if you change your book based on that agent's feedback, you might have inadvertently made it less appealing to another agent, while still having no chance with the first agent. So the individualized feedback can be a mixed bag.
And then you get an agent who said they thought your book was wonderful but it's still a rejection. So then you think, Then why reject it?
Despite all these issues, however, I still always hope for something personalized. I completely understand your frustration. But lots of times you'll get a form reject for a full.