There's a matter of practicality and sense, plus a large chunk of reason, StellarRat. The amount of force that a fifty caliber has is just much much more than a helmeted guy is going to survive, unless you want to start assuming truly insane things about the situation. Ballistics are ballistics are ballistics.
Also, you'll note that all versions of the M1 tank carry a M2 .50 in the commanders cupola, in addition to the main gun (which is actually a 105 rifled in the M1, only being upgraded to the 120 smooth-bore for the M1A1 and M1A2). You will kill the absolute hell out of anyone dismounted that you hit with that machine gun, it's designated purpose is to engage light armor, after all.
Generally speaking, the main gun of an Abrams isn't to be used at long range against infantry (I'm full-time Army tanker, note), however, the difference in distance where you can hit someone with a .50 compared to a 5.56 compared to a 7.62 compared to a full size tank round should be obvious. Hint: the bigger rounds go further.
Apologies that I don't have the technical background that EchoFourDelta or Firebeard do, but long-story short, they're completely accurate.
