If MACO was the only military force of the United Earth, its purpose would have been defense of the United Earth. How could it defend it, if it could not even wage space battles?
MACO is a ground force. Ground troops (fictional or real) simply don't operate ships. That's what navies are for.
And yes, a space-based organization like Starfleet qualifies as a navy. Ships that operate in space ARE STILL SHIPS.