Why is 56k the fastest dialup modem speed?

As I recall, the USR Couriers could go 32kbit on bare wire / pseudo POTS lines.

There was some init string magic to make em ignore dial and ring and speak anyway; i forget its shape now. They'd talk to a sportster modem but those couldn't initiate, i think. and they didn't last through lightning storms like the couriers did.

I think it is due to clipping by the following multiplexed network stages that carry it. We know bare copper is good for a lot higher - not a phone pro.