Hahahahah I sometimes ask my self that too... But I will say although their samplers like the S-1000 may of only been 3U tall have you seen how massive they are in length?! Not light either...
Otherwise ask David Cockerell who designed all the Akai hardware
The S-2800 was filled up with air lol
Whilst the 3200XL is like some main frame server computer from the early 90's lol
At a guess they couldn't be bothered to redesign the boards when going in the S1000kb and stuck em all in there so had to make the case massive to fit it all