There's no demand for them. PC133 RAM hasn't been used for several years, even in the Mac world. And Apple was one of the last computer companies to adopt DDR RAM - it had been used in the PC world for a couple years prior to Apple's implementing it in mid-2002.
Companies could make PC133 1 GB modules, but no one would buy them. Not worth it. In fact, it's unlikely that we'll see standard (unbuffered, non-ECC) DDR DIMMs exceed 1 GB, now that DDR2 is out.