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.
It all has to do with the memory controller chips built into the machines. A memory controller can address RAM up to a certain size, and no higher. Beyond that point, it cannot "count" the rows and columns on the memory chips. This is why you can't install a 512 MB PC100 DIMM in a G3 or G4 "Yikes" motherboard for example, even though the module exists, the memory controller is incapable of using it.
So if the population of machines that take the format of memory (PC133 in your example) cannot address the high density chips you'd need to construct a 1 Gb module, then there is no market for the 1 Gb SODIMMs.