Ultimately, the answer is fuel density. So, for long distance untethered travel, like planes. Beyond that, it's plastics production and chemical manufacturing.

We can switch to hydrogen for lots of stuff that requires carrying your fuel on your back, but some things get tougher because the density is just not the same as a hydrocarbon.

These are all surmountable (biodiesel, carbon capture->fuel cycles, bioreactors, etc), but they take time and money.

In the end, what will push us to get there are economic shocks. We're getting there, it's just painful.