A reasonable question.  Everyone has their reasons.  Sally Kane provides 15 in her article on about.com (linked here), some of them more compelling then others.

I have my own reasons based on my 10 year experience at a big firm.  Here are my top five:

• Generally, it is easier to go from a prestigious, big law firm to a small one or anywhere else (government, in-house, etc.).  Note that I am not saying that associates can never transition from a small firm to a big firm (and one of my friends has achieved this feat and he will tell his story on a later post).
• The training is generally by partners who are at the top of their field (including those well-known in the area due to government service, teaching, speaking, or prolific writing) and can serve as excellent mentors.
• Big firm associates have multiple opportunities to learn the business side of practicing law (including charge-offs, collections, client management, etc.) which helps prepare them for life there or at smaller firm (or even their own firm).
• Big firm associates can generally specialize in an area (corporate, securities, tax, etc.) and become an expert in that field (by concentrating on certain projects, assuming the work exists) and, in some cases, have the opportunity to develop a sub-specialty in the area.  Note that this specialization can happen at smaller firms as well but the structure of a big firm with large clients may provide this opportunity more readily.  
• And related to the fourth one above, big firm associates may have many speaking/writing opportunities early in their careers given the firm’s big presence.