When a combustion engine is idling, it’s usually not burning anywhere close to 100% of the fuel that it draws, which means higher quantities of unburnt fuel are

When a combustion engine is idling, it’s usually not burning anywhere close to 100% of the fuel that it draws, which means higher quantities of unburnt fuel are reaching downstream parts of the engine system (like the exhaust system) that are only designed to be moderately resistant to fuel.