Because, in the UK justice system, the punishment is based on the crime, not the criminal, and percentage fines are not based on the crime.
Would you also support basing punishments on other irrelevant factors to the crime such as gender or sexuality?
No, because they are completely irrelevant. Punitive fines works because they punish and therefore act as a deterrent. If you can afford the fine without batting an eyelid, the deterrent doesn't work. For offences like these I think means based testing is highly appropriate and the only way to ensure that they equally punish / deter everyone.