The question is posed with the assumption that RS-232 was always ±12V.
Unfortunately, the short answer is, it was not always ±12V.
The specification requires that an RS-232 driver outputs a minimum of ±5V and maximum of ±15V when connected with the specified load of one RS-232 receiver. The output is allowed to have up to ±25V when no receiver is connected. ...
There are maybe a few points left out here.
RS-232 was first recommended in 1960.
That's long before there were low-voltage devices. In fact, it was originally designed to work with electro-mechanical, not electronic devices, like TTY.
RS-232 uses directed voltages.
So it's not simply 0V and 12V but +12V and -12V. Quite useful to work with coils.