It's because it is ineffective and doesn't have any consequences if they fail. Withdrawing will prompt blue states to push even harder om regulation, as well as the EU possibly waking up. I think they are afraid of the Streisand effect, nothing more.
Look at good guy Exxon sticking up for the planet /s
It's an easy PR win for them, cause the Paris Agreement is more symbolic than anything, as it is woefully ineffective at actually punishing countries for failing to meet targets, so if Trump chooses to stay, they can milk the environmentally friendly PR and the status quo continues.
It actually hurts them if Trump chooses to leave cause it puts them into the spotlight.
My take is they've already established work arounds and if it changes then they'll loose that leverage, and potentially have a worse deal to work with in the future....not that any Dem will ever hold the majority or oval office again, but maybe.