If they're not on the drive, they're definitely on one of the install discs, although you might have to download a hefty update.... there should also be an entire developer tools installer package on the disc. It must be easy to find, because 99% of the people who come here trying to install X11 install the X11 SDK by accident instead, and it's in the same installer package, I think. 😱
I'm not 100% sure, but I think that dev tools package is on disc 1 and that XCode is on disc 2.