That’s true, but there is far more energy to gain by fusing hydrogen compared to carbon. I’m not sure how it compares to uranium though. I suspect it might be similar. (I mean, obviously in practice you wouldn’t / couldn’t actually get the energy from fusing carbon - but we can still compare hypothetically. … also, I’m sure we could get a clear answer by looking it up; but this is one of those things where thinking about it is probably more interesting than knowing the answer.)