Kevin as a non american I think I can explain this without problems:
Desert Storm 1990: 2900 Abrams, over 3500 allied tanks, 2005 I think US Army + Marines have around 4000 M1 in comparison, Uk army has (if I'm not wrong around 400 Challengers and the for french army with the Leclerc).
everything is clear...
Add to that the US Navy with at least 99,9% carriers of the world (french Navy is the only one to got a nuclear carrier and it has 1 (real carrier I mean not the smalls ones UK has for his ADVs))
Add to that the USAF and I think for each poor MBT of the world there are so many US MBTs and aircrafts that during a war I will disapear witout firing.
And don't forget it's the game...
More seriously:
First "turn" : M1A2 vs Challenger2:
4 rounds, kill report (M1A2/Challenger): (13/4),(13/5),(13/8),(13/8)
My explanation: hull armor of the Chall 2, as for the Merkava
Leclerc vs Leopard2A6-EX:
4 rounds, kill report (Lec/Leo): (11/13),(4/13),(2/13),(4/13)
Quite unpredictable, maybe because of the size of the Leclerc
Merkava vs T-90:
very ennoying, they were unable to kill each other at that range... But I'm sure Merkava would win
Leclerc vs Merkava: only one round, 13/0
Challenger vs Merkava: only one round 13/0
I precise that fight started with a first move from one side which I swaped each time...