The reason the drive ratio is changed in the transmission and not just the rear end is all for packaging. The T56 trans commonly has 2.97:1 first gear ratio, (though there are 3 or 4 others available depending on application), with a 3.42:1 rear end gear, you get 10.15:1 final drive,
If the 6th gear were 1:1, then the rear gear would have to be 1.71:1, which to achieve the same first gear final drive would require a 5.94:1 first gear. Basically you would need a larger transmission case to house the gears, which would then take up more interior space, or eliminate any ground clearance.
There would also be an issue with a 1.71:1 rear end gear, you would either need to use a VERY large pinion gear (not a good idea), or a small ring gear (would limit power handling capability).
As a side note, If I could afford a T56, I would put some 4:56 gears in my truck to take advantage of it. (I don't see much point in having your cruise speed be at 1200RPM)
