Sobers was selected a bowler first too, incidentally.
The thing is that batting is something that you can be good at even without a lot of natural flair or technique, through constant practice and with solid concentration. Experience plays quite a large role too, there are so many batsmen out there without the best technique who are able to put up big scores through simple grind, concentration, and knowing what shots to play and when.
Bowling on the other hand is something which is harder to pick up later on - a bowling action is after all more un-natural that thwacking a ball with a bat. You usually need to start bowling from a young age to be able to do it to any degree of competency when older. It also takes a lot more practice to develop the accuracy and variations needed at the higher level, it's less about experience than batting; and more about constant practicing of skills and fine-tuning the bowling action.
It is also more strenuous on the body than batting, which is why many allrounders give up bowling as they get older and focus on their batting.
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