A6 and A7 are connected to the ADC input multiplexer only, not to any of the digital I/O ports.
PORTC, the I/O port used for pins A0..A5 also interfaces to the RESET pin (if the chip is programmed in a particular way), so its not available for supporting two more pins. The Arduino setup keeps things simple and only supports the RESET pin as a reset input.
More details available on the ATmega328 datasheet. [ a lot more details! ]