Other licenses like the LGPL and BSD allow you to create proprietary software, so don't help the free software community as much.
LGPL still helps the free software community quite a bit. If a proprietary vendor decides to use an LGPL library in there closed-source game/program they may find ways to improve the LGPL'd library and they will submit those changes to the developers. This allows their game/program to stay closed-source, but still allow for improvements with the LGPL library.
Also, LGPL, BSD, etc libraries are more likely to be used by aspring game programmers and indie game programmers because they cost nothing. This contributes to the popularity of the the library which indirectly contributes to the amount of documentation and other resources available which makes it easier for the next guy to learn.
Using GPL for libraries will just mean that they'll only be used by hobbyists. LGPL is a much better choice. Not using BSD is fair, though. It's a tad too free for my liking, in many cases.
is a great example, however, of a BSD-like-licensed product that still is a valuable asset to the community. It's used by many big name software companies. (Ever used Adobe Acrobat Reader? If so, you've used something made using Boost.) The license basically boils down to "do whatever you want with it in compiled form, but you can't remove the copyright notices from source code".