Historians know more about individual aircraft in WWII because there were fewer of them and because air forces kept more detailed combat records in comparison to ground forces.

A rough ratio of combat aircraft to ground troops for the early portion of WWII is about 500-1000 servicemen for each combat aircraft. Looked at another way, this is about 20 to 40 aircraft for each division (10,000-18,000 men) of ground combat troops. By the end of the war, the allied forces would greatly increase number of aircraft relative to ground troops, but ground troops are still by far the largest numerical component in a WWII military.

Additionally, WWII air forces generally created detailed records. A ground force commander can send staff officers to see what's happening on the battlefield, but an air force can not understand the combat situation except by asking pilots what happened. This process was called debriefing and generally occurred after each mission. The debriefings were recorded by the air unit intelligence staffs and used to create a written account of each mission so that air commanders would know what happened. This process leaves a lot of written records, and a great many of them survived the war and are accessible to historians. Of course, the eyewitness accounts of aircrew are not 100% reliable. Long missions, combat fatigue, disorientation, navigational errors, human failures and so forth mean they need to be carefully interpreted. If after the war historians have both the Allied and Axis air force records for a given time and place, a painstaking reconstruction of what most likely happened on a given mission can be created. For individual battles of special interest, this has often been done, but for many WWII aerial encounters it has either not been attempted or there are simply not enough records that survived the war.

The record keeping process for ground fighting is much less detailed. Firstly, it's rare for a front line soldier to get to write down his experiences in a timely fashion. There is no safe airbase for him to return to after action, he is likely to spend a few nights under shellfire in a wet foxhole before he gets a chance to write down his account, if indeed he ever does. Therefore the daily record keeping was conducted by the staff of higher headquarters. Often these daily records are called the "war diary" of a unit. Divisions and higher headquarters usually kept a war diary, Regiments/Brigades (1000-5000 men) sometimes kept one, and for Battalions and lower echelons, it's uncommon. So the daily mission records that we have for individual fighter pilots don't exist for infantry units until we get to formations containing something like 1000 soldiers or more.

However, even with the tendency of air forces to keep good records, it's fairly uncommon to be sure which pilot shot down which enemy aircraft. Axis records have not always survived the war. Soviet records are not always accessible. Often the records of each side fundamentally disagree about what planes were seen where and shot down what. The western bomber offensives against Germany are particularly good for abundant records and historians who can access them, most other aerial theatres of WWII are more difficult.