Why are (galley) proof requests given such a short deadline? Often the publisher requests to get the proof within 24 hours when it's ready. What are the reasons for making this so short? Do they want the authors to not make too many changes?

EDIT:
The email I received said:


  Please ensure you check the entire article carefully, and answer all
  queries. Return corrected proofs and any related material by uploading
  to the site within 24 hours.


EDIT: @StrongBad pointed out a related question.
Galley proofs are part of the production process where a book or journal issue is actually printed, as opposed to the 'softer' process of deciding what pieces will go into it and in what form. As such, the deadlines for their revision are associated with the physical production process rather than the editorial process for the piece and can be quite different from deadlines for e.g. minor revisions or revise-and-resubmit requests.
These proofs are only meant to be used to check that the typesetting correctly represents the author's intent, and not that the content is scientifically correct (which should have been done at an earlier stage). Occasionally a one- or two-sentence 'note added in proof' may be appended to a paper but that's about it; for an example see the AIP style guide, p.11. Checking the typesetting is assumed to be a straightforward matter that does not require more than one day (though assuming that an academic can spare the time at the publisher's decision with no prior notice is another matter), so such deadlines are usually OK.
Note also that such deadlines can be negotiable if properly handled. If such a requests lands on you and you will not be able to complete it in time, it is usually acceptable to notify the editor, as soon as possible, that this is the case. A polite note along the lines of

Dear Editor,
We have successfully received the proofs of our article. Unfortunately, today is my thesis defence, my coauthor is getting married and my advisor is away due to travel, so we will be unable to complete your request to review the proofs within 24h. We will get them to you as soon as possible, which will likely be the day after tomorrow. Is this acceptable, or will it lead to a delay in publication?

can work wonders in stretching such a deadline. From personal experience, I have seen a 24-hour request be stretched to a full week without a publication delay.