Why can All Might still do his buff form?

I AGREE ALL mights superior strength does not come from his quirk,it comes from his killer workought.This was shown when he fought wolfram.He has his quirk but in a drained form .He has it but he cant unleash it.

All wrong. All might in his past was always a big buff guy thanks to his training. He never had a "skinny form" his muscle form is how he used to look always before AFO injured him. As all might said to deku, he wasted away from his surgeries and complications. When he uses his quirk his body transforms to its former prime to handle the power. As for why he can still do that form for brief moments may be for comedy purposes or maybe the quirk doesn't fully leave the host after passing it on. If so, hopefully all might will be able to utilize this in a way who knows.

All Might's buff form has nothing to do with One for All. He mentions at the very start of the series that it's a conscious effort similar to flexing. Just because All Might lost One for All doesn't mean he also lost the ability to flex. He just can't keep it up for a long time on his base power.

If you are asking about latest episode then it's taking place before the events of 3rd season.

Yes, it's mostly for comedic purposes. Some can argue that he still has a tiny bit of power but he used all of One for All. It's clearly stated in All Might vs All for One fight.