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They've been around for a really long time. I suspect that back in the analog radio days of yore, the control pulses transmitted by your Futaba were recieved as a pulse train - 6 channel radio, six pulses and a big enough gap between frames to reset the index. At the RX all you have to do is feed those pulses through to successive servos one after the other, which is easy and cheap to do with basic logic ICs.

So when you use hobby servos for robots, you're taking a low precision actuator meant to make a flap or throttle or other control surface go relatively up, down, in, or out, that was designed in like, the 70s, and asking it to do modern robotics stuff