Hardware

Because there is no need from an address space or compute standpoint.

to understand how large 128bit memory space really is; you'd need a memory size larger than all the number of atoms in the solar system

In the rare cases where you need to deal with a 128bit integer or floating, you can do it in software with not that much overhead by concatenating registers/ops. There hasn't been enough pressure in terms of use cases that need 128bit int/fp precision for manufacturers to invest the resources in die area to add direct HW support for it.

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FWIW there have been 64bit computers since the 60s/70s.