It is because you have to use Mesa, which might be running in software.

Mesa is an awesome package by "Brian Paul"mailto:brianp@avid.com which "emulates" OpenGL.

Basically, it is a fully compliant OpenGL package, but it isn't officially sanctioned by the OpenGL ARB, such, it is doesn't have the OpenGL name.

There is an acceleration package for the many graphics card, but I don't know anything about it.

nVidia is being very supportive of Linux: their TNT2 and GeForce cards work under Linux with hardware-acceleration of openGL. (but the drivers are proprietary)

radeon cards should also be supported very well under linux (even with open-source drivers)