The problem is that we need it to just barely break even and then a little bit more. Having a tremendously energetic fusion device is both useless and a solved problem: we already know how to light one off, but it only lasts a millisecond and it destroys everything for miles around.

For what it is worth, fission also suffers this problem. A promptly critical fission reaction is possible, indeed quite easy to achieve, and very undesirable. All fission technology is dedicated to keeping the reaction just barely critical.