From my reading of OP's question, it sounds like he's asking, if Latex as an overall collection of software/ utilities were made by one person or group, why would they choose to put PDF titling in hyperref, a package seemingly with the more concise goal of providing hyper referencing?
So, the best answer might be to point out that Latex is a collections of SW/ utilities/ tools made by many independent groups, usually working without a higher-level organizing structure.
So, in this case, I suspect that kind of like Alan said above, whoever made hyperref probably felt this PDF titling/ author listing functionality wasn't easily accessible in any other package, and figured that since they knew how to do it anyway, they'd just throw it into hyperref.
It's not that somebody explicitly said "I'm going to put PDF titling in hyperref rather than some other more obvious package."
hyperrefpackage is written by Sebastian Rahtz and Heiko Oberdiek, although I think it's mainly Heiko's project now, so there isn't really a "they". Many of his packages make use of specific properties of pdf, presumably because of his interest in and knowledge of pdf, and that's probably why he put it into the package. – Alan Munn Aug 25 '11 at 01:11hyperrefpackage. See How can I generate PDF metadata from LaTeX? that describes using\pdfinfo{...}with/<meta-tag>operations/definitions. – Werner Aug 25 '11 at 17:24