We need decentralised miners who maintain computers. We can have decentralised taxes who will take care about environment. I thought it is easy to understand
What happens if both groups decide to increase the tax rate? Who decide which groups can increase the tax? Who decides how many trees or penguins are enough?
What happens if I don't pay? Who will force me to pay? Sooner or later you need some tanks to collect the taxes, just in case. Tanks are centralized, and that's the government.
Actually, each region can decide to have it's own group of tanks and fix it's own tax rates. Just like now.