Speaking as a former Design Director, Typographer, and Production Manager of many publications and also of national-market print advertising work:
The reason that there's a tight deadline for authors' galley proofs is because of what galley proofs are for: evaluating whether the formatting has introduced any issues with readability or meaning; whether there's any typos or format errors; whether there's any omissions or duplications.
The turnaround is tight because it's part of the production phase, not part of the editorial phase. The time to edit and re-write and fuss over the article is done and gone. Galley proofs is a final reality check, not a chance to revisit that awkward sentence in the 4th 'graph.
Traditionally in print, editorial and not production is given the luxury of extra time. Usually there is no luxury of time, in spite of what it appears to the author. Most journals have a lot more production steps to go through and are very close to press time when the authors' proofs go out. It may seem like "not a big deal," but a printing operation has scheduled their presstime very closely, and if your book is late, it gets bumped from the schedule in favor of something that is actually ready for press. If your book is bumped from the press schedule, it might be days or weeks before it can slot back in. The cost to "hold" the press is spectacularly prohibitive.
Production and pre-press times are shrinking these days, it's easier today and faster to get a book to press than it was in, say, 1985. In many ways that exacerbates the problem with proofs turnaround...there's just no "fiddle" time anymore.