I don't think it's about operating a machine per se, it's about the danger level of the machine. As others have pointed out, there's been no attempts to regulate operating a bicycle.
A car requires a license because it's a particularly dangerous machine, obviously not all machines require licenses. A car passenger could cause a car accident (have you seen the video of a woman attacking a bus driver, causing the bus to fall off a bridge, killing 15 people?) Yet we only require the operators of such machines to be licensed.
