Because it creates a situation where it becomes stupidly risky to be affluent; people do make mistakes with e.g. speed limits - not everyone crosses them intentionally - and paying a € 50 000 fine for doing 21 km/h over a speed limit really makes people consider emigrating and taking their businesses to a more reasonable place. It also makes it more difficult to attract high-income workforce from abroad, thus having a negative impact on high-tech industries.

It just puts more pressure on people to move their wealth into corporations. How would you fine someone with no income or wealth, but who controls corporations that have plenty?

Also, a persons wealth has nothing to do with the crime. Punishment should be based on the crime, not on a person's status.

If person A and person B commit the same fineable offense, it seems fair to me that the fine should be the same for both people. After all, shouldn't the fine be, in expectation, equivalent to the damage caused by the offense?