ATmega - Why is the prescaler factory defaulted to 8?
Note the dependency of the maximum allowed clock speed on the supply voltage: 
E.g. the 644PV can only reach 4 MHz when running at 1.8V (similar for other chips)

If the controllers were programmed to a default 8 MHz you could not program them in a circuit running at such low supply voltage. 1 MHz is a safe default frequency that any AVR can reach at any supply voltage within its specifications. 

You could change the internal oscillator to a 1 MHz one and leave the clock divider unprogrammed, but this forbids to run the controller at a higher clock rate without an external clock source.