Why isn't dumped fuel burned?
There's no benefit to burning dumped fuel, it actually introduces risk to the airplane which is dumping fuel and any other aircraft in the vicinity and possibly people on the ground depending on the altitude at which is done. Building a system to burn it is not worth the time, effort and money given there's no good reason to do it. 

Dumped fuel will vaporize and mix in with the air, and then break down over time from exposure to air, heat and UV light. This is not ideal for sure, but burning it isn't environmentally friendly either as it produces CO2, CO and soot. Evaporation of dumped fuel is a miniscule problem when looked at in context as it's such a rare occurrence. Canada measured 58 million liters of gasoline and diesel lost to evaporation from retail outlets and pump in 2009, if you extrapolate that to the US it's got to be over 100 million US gallons of fuel evaporating into the atmosphere every year. An airbus A380 contains 320,000 liters of fuel, if you dumped all that at once it's not going to make a statistical difference.