Probably because 76mm is good enough for infantry support and killing most German tanks or vehicles?
Allied 76.2mm had a 'shatter gap' problem. Rounds with too high an impact velocity would sometimes fail even though their penetration capability was (theoretically) more than adequate.
This was a problem with the British 2 pounder in the desert, and would have decreased the effectiveness of U.S. 76mm and 3" guns against Tigers, Panthers and other vehicles with armor thickness above 70 mm.
The nose of US armor-piercing ammunition of the period was 'soft' (brittle?). When these projectiles impacted armor which matched or exceeded the projectile diameter at a certain spread of velocities, the projectile would shatter and fail.
For the a 76mm APCBC M62 the shell would shatter and fail between 50 meters and 900 meters. These ammunition deficiencies proved that Ordnance tests claiming the 76 mm gun could penetrate a Tiger I's upper front hull to 2,000 yards (1,800 meters) were sadly incorrect.
Here's one story of 2 Tigers being engaged by a mix of Soviet T34/76 and US lend lease Shermans:
The 13.(Tiger) Kompanie, of Panzer Regiment Großdeutschland, reported on the armor protection of the Tiger: "During a scouting patrol two Tigers encountered about 20 Russian tanks on their front, while additional Russian tanks attacked from behind. A battle developed in which the armor and weapons of the Tiger were extraordinarily successful. Both Tigers were hit (mainly by 76.2 mm armor-piercing shells) 10 or more times at ranges from 500 to 1,000 meters. The armor held up all around. Not a single round penetrated through the armor. Also hits in the running gear, in which the suspension arms were torn away, did not immobilize the Tiger. While 76.2 mm anti-tank shells continuously struck outside the armor, on the inside, undisturbed, the commander, gunner, and loader selected targets, aimed, and fired. The end result was 10 enemy tanks knocked out by two Tigers within 15 minutes" (JENTZ, Thomas L.; Germany's TIGER Tanks - Tiger I and II: Combat Tactics; op. cit.).
Here's an image of a Panther after being struck by multiple 76.2 mm rounds:
Die Tiger FibelOne thing as well, if the first round doesn't penetrate, chances are none of the following rounds will. Firing from the same range, into the same spot, will not 'wear a whole' in armor. If you stay in one spot firing over and over then get killed thats not the AHs gv modeling, that's your own stupidity. The facts are that there is a lot of 'randomness' when it comes to 'knocking out' mbts. More so then you see in AH. If you go head-to-head vs. a Tiger in a T-34/76 you will die and that's all you need to know.