It's not an issue with the concept of fusion, just the particulars of the stars. Fusion in other contexts is vastly more intense.
"The power output of the core of the Sun is about 276.5 watts per cubic metre — that's almost three of the old 100W light bulbs. On a power/volume basis, it's a lot less than your body emits (about 100 W) and around the same as a compost pile.
How can it be so low? After all, a relatively small hydrogen bomb such as the American W88 has the explosive power of just under half-a-million tonnes of TNT — and fits into a volume smaller than a rollaway wheelie bin. So, if the Sun does nuclear burning, how come it generates less power (volume for volume) than a hydrogen bomb?
The answer is surprising. The Sun does do nuclear burning of hydrogen atoms, but only very occasionally. How occasionally? On average, any given hydrogen atom will run into another hydrogen atom only once every five billion years. The Sun's core does do nuclear burning, but very sparsely, with the reactions few and far between. Luckily, the core of the Sun is enormous, so even though it's only as powerful as a compost pile, there's enough energy given off to keep our planet warm."
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