Because a powerplant has one extra required component that a rocket doesn't- convert the reaction energy into electricity. Every known, engineered method doing that that we currently have is sufficiently inefficient that more energy is lost in conversion than the fusion reaction produces, which is why you get less usable energy out that you put in. It's easy to make energy from fusion, it's hard to get that energy in a usable form, which is what we mean by "break even". A rocket is not subject to that constraint, because the raw reaction products themselves, without conversion to any other form, are exactly what a rocket wants anyway.