Because in real life, a single bullet can mean life and death. A 7.9mm can do a lot of damage that is not shown in AH and just the fear factor of constantly having rounds being shot at you was a big deal. The cannons were more effective, but planes could only carry so much. By replacing 1 cannon for 2 MGs, a pilot had more firing time and more chances to do damage. In AH, we do not have luck as a factor in the damage models and no partial damage. A single 7.9mm could rip a hydraulic line or kill the engine if well placed or was a lucky round. In AH, a single 7.9mm is nearly useless unless aimed DIRECTLY at the pilot.
Also, there were plenty of ground targets that did not require high caliber rounds to cause damage. Trucks, trains, barges, cargo ships, and infantry were all attacked. In AH, we use a point system for damage where once you pass a "point", the object is destroyed, therefore, cannons are a much more suited to ground attacks. Again, in real life, a single 7.9mm can cause a lot of damage and a lot quicker with a higher RoF.