There are definitely places where a faster turnaround from final PDFs to ready-in-warehouse would be appreciated. Most of the work my company does is for educational publishers (in the UK), where time is often very sensitive. New exam board specifications come out, or changes to the national curriculum, and teachers need books to cover these changes by the start of the new academic year. In an ideal world, they actually need them at the tail end of the previous academic year, because that is when department heads make their decisions about which courses to follow (having books and other resources already available for a course can swing that decision).
The process has already speeded up considerably since I started working in educational publishing 8 years ago. Then, the turnaround time was closer to 8 weeks, because of still using film and the like. These days the majority (I'm no longer at the mine-front, so I'm not absolutely certain this is the case in all educational publishers) of books go to the printer as PDF files, and they can print directly from them. This cuts out some of the time in the process. I believe the turnaround time is now closer to 5 weeks from finished PDFs to ready-in-warehouse (of course, you've then got to send the books out, which works differently in educational publishing, as the majority of them go direct to schools, rather than to bookshops). The actual printing can take as little as 1 week, depending on the size of the print-run.
However, as others have already mentioned, it's not possible to do this for every book. A publisher has to pay a considerable premium to a printer to have them print a book that quickly.
Why? I can see why it would seem strange that it's not possible to set this exact speedy time for every single book. If you scheduled it properly, surely it would be possible? Perhaps one day it will, but I can't see it happening very soon. Because you have to build in contingencies. Problems happen. And they happen a lot in publishing, for some reason! As someone else mentioned above, this may be due to the large number of people involved in the whole process. Delays have a tendancy to slip in at some (or even all) the stages. It could be at the stage when the author is doing rewrites; it could be at the editing stage; it could be at the typesetting stage; it could be at one or all of the proofreading stages; it could be due to illustrators, photographers, designers... there are so many people, departments and external suppliers involved that it would be extremely difficult to get the finished PDFs to the printer at the exact moment they would need it and when their press had been cleared for that specific book.
Why do problems and delays occur? Because pretty much everyone in the process is working on a large number of projects; not just this one. A freelance editor or illustrator might be working on 4 or 5 projects at any one time (some a lot more); an in-house managing editor may have 10-20 (again, sometimes a lot more) projects that they are managing at any one time; a typesetting company or design studio may be working 20+ projects at the same time... and so it goes on. Someone has to prioritise somewhere. We offer faster turnaround to publishers, for an extra cost (a considerable extra cost, because setting aside time specifically for one project that might slip, means we could end up sitting around twiddling our thumbs for a week with nothing to do); as do other suppliers. But it can't be done for every project. It would only be possible if there could be an absolute guarantee that the job would turn up exactly when it was booked in for. Which you just can't guarantee in this business. At least not at the moment.
The trouble in educational publishing (in the UK, at least; it may be different elsewhere), is that almost every single book (or resource pack or electronic resource) is time sensitive. Every list (e.g. primary science, secondary science, primary maths, secondary maths, secondary English, secondary Modern Languages... etc. etc.) has books that aboslutely have to get out by July (or by September). Are the English books more important than the science books? Should the Modern Language books take priority over the maths books?
Amazingly enough, most of the books do come out on time. But this is done by various people building in a bunch of contingencies - i.e. more time in the schedule that should in theory be needed.
So, in theory it should be possible to speed up the process a little bit more, but it is less to do with technology (which has already speeded it up considerably) and more to do with a culture shift and possibly even a considerable increase in in-house staff, which would be unlikely to be economically viable. Maybe one day, but probably not any time soon.
One technological elememt that could make a difference in educational (and academic publishing), however, is electronic delivery of all the books, rather than printing them. Education is particularly suited to this technological change, IMO, and I think that in the next 10-20 years, more and more teaching and learning materials will be delivered electronically rather than in print. It's already happening to a fairly small extent, with many educational publishers providing some material on-line. This allows for much quicker updating of material (and completely cuts out the 5 weeks or whatever for printing, plus the 2 weeks or whatever for distribution). With the right hardware (e.g. e-book-readers of some kind for all pupils/students; electronic white boards in every classroom, etc.) this could prove more useful than printed books. It's not going to happen overnight, though. And it's (probably) not really relevant for other areas of publishing. It's going to be a long time before printed novels become extinct, even though there already some people who do prefer e-books to printed ones.
[Sorry to go on! Hopefully that gives you a bit more of an idea of how it works - at least in my particular corner of the book world!]