While this question has been raised and answered many times, I'll state it again (no harm done there).
Ghost Rider's 'fire' isn't just a conventional fire where you light a match or turn on a stove; it's a mystical type of fire called 'Hellfire' that 'burns' the victim according to their sins (the intensity of the 'burn' depends on the severity of the sins, that's why GR has an extra % to place a judgement on a villain opponent, because of their sinful ways).
This is why he's not immune to incinerate; it follows the comic book lore.
If you need further clarification then Google 'ghost rider hellfire' and your questions wil be answered there.
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It make sense
Ghost Rider's 'fire' isn't just a conventional fire where you light a match or turn on a stove; it's a mystical type of fire called 'Hellfire' that 'burns' the victim according to their sins (the intensity of the 'burn' depends on the severity of the sins, that's why GR has an extra % to place a judgement on a villain opponent, because of their sinful ways).
This is why he's not immune to incinerate; it follows the comic book lore.
If you need further clarification then Google 'ghost rider hellfire' and your questions wil be answered there.