Because he's an injury prone,uncommitted bowler who doesn't have a good record against the two top sides of his time.
I'll break your post into three bits.
Injury prone is irrelevent while judging the peformance of a player.
Judging a player's commitment is not only completely subjective, but again irrelevent if we are only concerned with actual performance, pretty much the only benchmark that makes sense.
And I am not quite sure how you came to the conclusion at the end of your post. His record against South Africa is, if anything, better than almost any other Pakistani bowler in our history. And from a perosnal prespective, his spell at Durban in the 98 series, as well as the variety and guile he displayed in the 03/04 home tour account for some of the best bowling performances I have ever seen.
His record against Australia, on first glance, does seem pretty shoddy. 31 wickets in 10 games isn't really the hallmark of a great bowler, is it now. However, this is where depending purely on statistics misfires.
Shoaib Akhtar, on more than one occasion, has destroyed the Australian batting line-up like no other bowler, with the exception, perhaps, of Shane Bond. Colombo 02 and Melbourne 04 are prime examples of that in test cricket. He has frequently won personal battles against Lehmann, Gilchrist and Hayden, with Ponting being the only player, if my memory serves me correctly, that has clearly gotten the better of their mutual encounters.
And the fact that he has come up with fast, furious and feisty spells on wickets which offered little to no conventional support to the bowlers only serves to illustrate his achivements.
Of course, Shoaib Akhtar, at the beginning of his career was not the bowler he grew into during his later 20's, early 30's. However, his performance over the last five/six years, on
extremely unresponsive wickets and often in extremely unsporting conditions against some of the best batsmen in modern cricket, with all their fancy gear and without the aid of friendly home umpires is unparralled in Pakistani bowling history.
There just are too many spells that come to mind.