Why All for One never stole One for All? All for One is the most powerful superhuman with the exception of All Might who has the quirk One for All. If All for One has the quirk who makes him able to steal quirks, and All Might is the only person he can't defeat, why All for One doesnt steal One for All and let All Might quirkless to be able to defeat him?
First, What is One For All?


  One For All is the union of two Quirks: A Quirk that stockpiles power, and a Quirk that could be transferred to others. This mutation was caused by the Quirk, All For One.


One For All cannot be stolen by force: 


  One For All will not transfer to a new recipient unless its user wishes it, thus One For All cannot be forcibly stolen. For example, the Hero Killer Stain ingested Izuku's blood, but did not inherit One For All because Izuku did not wish it. However, this does not mean One For All cannot be forcibly passed on.


Why All for One doesn't steal One for All and let All Might quirkless to be able to defeat him?

Apart from the reason why can't One For All be stolen, you can see that:


  After One For All is transferred to the recipient, the previous One For All user can still use the "leftover embers" of One For All, and thus can still access One For All's power. However, these leftover embers will not last for long, and the previous One For All will slowly run out of the embers, resulting in the previous user losing One For All completely.


All Might has the "leftovers" of One For All but soon or later he will loose One For All completely, the user with One For All now is Izuku, if the Quirk was to be taken it had to be from Izuku and not All Might.

Source: Wiki