For the same reason they're happy to burn the only habitable planet humans will ever have: they're going to play with fire in search of optimal profit, even to their own eventual detriment.
People act like raw, naked, decayed capitalism is a totally stable system where all actors work perfectly together to rationally suppress the proletariat in perpetuity. But it's not that. Capital has accumulated to the point of a handful of individuals butting heads over all of the earth's resources.
There's no longer a cabal of dozens of cartels coordinating in abstract lock step. There's like 10 dudes that can meet in a room. Applying dialect-materialism to that is using Newtonian physics on a quantum scale.
Petty grudges and irrational emotions take control at that scale. A few of them get the idea that they can eat the others by leveraging the proletariat and all bets are off.
Will it certainly happen? Not necessarily. But more individuals have deposed through creative leveraging of legal mechanisms than those spontaneously pulled from their beds and shot. There's no reason to not be prepared if the opportunity presents itself.
Also, petrochemicals are a requirement for any sort of transition to alternatives. Humanity has been binging for nearly two centuries on free energy through hard oil dependence.
If we don't tirate off of that the shock will (functionally) kill us. The thought that halting that consumption will solve everything is like an extreme alcoholic thinking quitting cold turkey will work.