Mic,
I think that's because most of the common bike setups are similar to kit car/Z-cars installations and space isn't an issue (who needs back seats?!). Plus the advantage of moving the engine forward is better weight distribution.
Stuey,
Think you're right: There's no reason why you shouldn't be able to make up a chain diff cradle below the engine mounts (might have to be selective with the engine choice and look at dry sump etc to keep the deck height low). Might also limit ground clearance a bit because you could end up with the driveshafts offset down.
If you're ok with a welder and reasonably practical... nothing's impossible.