Merkava carries troops internally as passengers not as riders.
Active antitank missile systems is a good enough reason to prohibit "riding" the tank. Higher speeds of MBTs probably plays a role.
But I think that the turret design of most modern MBT is a factor you must take into account. Most turrets are relatively "short" in the vertical but quite sizeable in length and width. They cover most of the upper surface of the tank's hull. Add some "slopping" here and there and I think that you'll find that "riding" a tank isn't so easy as older tanks with smaller turrets and more available "space" on "deck".
As for the game ... as long as the Merkava "perculiarity" is already taken into account, and riding modern MBTs is not a wide spread hobby (
), no need to change anything. A scenario designer can always change the carry capacity and choose riders or passengers to suit his needs anyway.