The reason that infantry is underused in AoE is due to the numerous weaknesses of infantry compared to other units:
1) They're relatively slow, making them fall prey to Siege quite easily.
2) They are extremely vulnerable to massed Archers.
3) They are vulnerable to Priests (except for really large hordes of 'em).
Basically, there are three lines of Infantry in the game: Clubman/Axeman, Swordsmen/Legions, and Academy.
Clubmen and Axemen are primarily used for Tool Rushing, and it is relatively common to see Axeman rushes be quite successful.
Swordsmen and Legions: This line is comparatively useless in Bronze, due to the fact that they do not have sufficient protection against missiles and Cavalry in the form of Armour and Hit Points. In Iron, however, a certain phase is reached in which certain civs are able to dominate the battlefields by flooding them with swordsmen/Legions. The advantages to Legions in this phase is primarily that they are cheap and train fast, as well as provide the most amount of bang for the bucks of any unit in the game.
Hoplites: This line is, to some degree, even more useless in Bronze than Swordsmen, due to their extremely slow speed, which makes them even more vulnerable than swordsmen to missile units and Priests. When they do get their iron-clad hands on something, however, it GOES DOWN, and stays there. A few civs, however, can make effective use of Hoplites, if in the hands of a proficient player. Macedonian is simply the best for this; three of the primary weaknesses of Hoplites are greatly reduced, namely the vulnerability to Archers, Priests, and their small line of Sight. In addition, Macedonians have the relatively best Siege in Bronze, combined with Compies. Used correctly, this combo is darn near undefeatable in Bronze. Greeks, of course, have their marathon Hoppers. Basically, this makes the Greek hoppers far less vulnerable to Siege, and a slight tad less vulnerable to the other foes of Academy. Carthage has extra hit point hoppers, which it basically shouldn't use except possibly against Mace; in all other cases, it would be giving away a tough unit to the opponent.
In Iron, Academy is useful for its countereffect. Academy can effectively counter both Elephants and Scythes, and basically crush anything they get their hands on, except the higher-end Towers.
But the idea of making Academy units more vulnerable to Cavalry is just plain moronic. It would almost completely remove any usefulness Academy might still have in Bronze. The game is balanced and realistically accurate as it is, with Infantry being particularly vulnerable to Cavalry, wheras Academy is not, and will slaughter any Cavalry unit stupid enough to get close. It is as it should be.