I think relays are a lot slower in general. They are more binary as well, being either ON or OFF, where a transistor is actually amplifying a signal based on it

I think relays are a lot slower in general. They are more binary as well, being either ON or OFF, where a transistor is actually amplifying a signal based on its input (so it's possible to have a transistor output a range of voltages, between the current of its main source and pullup resistor being used to drag the off state to 0). Not sure how that affects in total, but I think using transistors lets you vary the voltage needed to drive the system without switching out all the transistors.