Define "deep"? AOW certs increase your limit from 60' to 100'. (from PADI at least) Deep Diver specialty increases it to the rec limit of 130'.
Why isn't it included in OW? Do you really think that most OW students are ready to be 100' underwater? Most can't even get life under control in 35' during their checkout dives! And before we get into the "PADI divides things up too much!" and "PADI makes you pay to dive!" argument...of course they do! So does every other educational body...you have to walk before you can run, right? You don't complain about not learning Organic Chemistry when you only paid for a semester of Intro Chemistry, do you?
The reason for the dive tables being all in one would be practicality. Does it really make sense to print 3 different sets of tables when you can conveniently put it all on one? Before I was started deep diving, I also found it interesting to compare NDL limits at different depths and really try to wrap my head around nitrogen loading...heck..it even made me go out and do some ACTUAL RESEARCH on my own! What a concept, eh?
Cheers,
Austin