I've recently been reading about T1 and E1 cables, which were used to transmit most calls inside and between telecom companies back in the day, and I was astonished that they transmitted data one sample at a time.

Unlike IP, those were synchronous, circuit-switched systems. You'd first use a signaling protocol (mostly SS7) to establish a call, reserving a particular timeslot on a particular link for it, and you'd then have an opportunity to transmit 8 bits of data on that timeslot 8000 times a second. There was no need for packet headers, as the timeslot you were transmitting on was enough to identify which call the byte belonged to.

Because all data from a single call always took the same path, and everything was very tightly synchronized, there was also no variability in latency.

This basically eliminated any need for buffers, which are the main cause of latency in digital systems.