computers don't fall asleep behind the wheel.
Sure they do. I've had computers go into sleep mode spontaneously. In the car, the analogue would be an equipment or software failure causing the system to believe it was meant to be switched off.
They don't get distracted fiddling with the radio, talking with passengers, or sending text messages
If they're smart enough not to give the self-drive system other things to do that it can get wrong and thus spend too much time on them and fail at driving, that's true. Even logging could fall into this category.
They don't have heart attacks or seizures.
Sure they do, at least, computer hardware does fail, just like our hardware.
Additionally, computers have more visual information available to them than humans.
Right up until the sensors ice over, or whatever. When your eyelids have ice crystals on them, you can blink a bunch and get partial vision back. This in particular is what is holding self-driving cars back now, and for that matter, self-flying planes. If the stall indicators fail for any one of several reasons then your only hope is that the pilot will feel what's happening and correct it. We'll have to turn vehicles into a rolling collection of redundant sensors to fix this problem, and the cost of the car will double.