>>39368448Probably because Game Freak focused on making the effects of the attack look so extravagant that they want it to be fully visible during the attack. Charizard's animation is so exaggerated that the actual blast would probably look obscured and janky as fuck if they anchored it to the mouth.
Don't mistake that for me saying I agree with the choice because I don't, but it's clearly a stylistic choice rather than technical ineptitude.