Compare the fines with taxes. Taxes are a source of revenue, fines are not. Fines are to discourage people from doing certain acts. The effectiveness of the fines should not be measured by how much revenue it generated, but by how many incidents were prevented. The decision to raise fines would be taken only if there's no significant drop in the incidents leading to the fine. For e.g. if parking offenses aren't going down, you may want to increase the fine for illegal parking. Once the parking offenses start going down, the fines are working as intended.

If you are looking at fines as a source of revenue, it becomes more of a tax, because the enforcement agencies will only be interested in collecting the money, not in preventing the incidents.
I would say that many enforcement agencies ARE more interested in collecting revenue then deterring an offence.

Speeding & Parking fines have gone up massively in recent years without the corresponding rise in offences. These fines are some of biggest revenue generating sources for the Councils & Police in the UK.