Because when you change a PCB trace from 0 to 1 or 1 to 0, the slope of the signal as it changes from gnd to v+ (the signal voltage) or v+ to ground isn't perfect, and that slope is highly affected by the various pieces of metal and silicon and fiberglass that make up the board and the chips. The shape and topology of the PCB trace matters, as do slight imperfections in the solder, PCB material, the bond wires inside the chips, etc. These effectively create resistors/capacitors/inductors that the designer didn't intend, which effect the slope of the 0->1 1->0 changes. So for these high-speed signals, chip designers started adding parameters to tweak the signal in real-time, to compensate for these ill effects. Some parameters include a slight delay between the clock and data signals, to account for skew. Voltage adjustement to avoid ringing (changing v+). Adjusting the transistor bias to catch level transitions more accurately. Termination resistance adjustment, to dampen reflections. And on top of all that, some bits will still be lost but because these protocols are error-correcting, this is acceptable loss.

This is how people were able to send ethernet packets over barbed wire. Many bits are lost, but some get through, and it keeps trying until the checksums all pass.