A fusion reactor, like the ones you hear about in the news, is a device that fuses together special kinds of atoms of hydrogen called tritium. Those fusion reactors are only designed to fuse very small amounts of those atoms at any given time. For example, the excess energy generated by the recent National Ignition Facility, that recently made headlines, was 0.4 MJ. That's just enough to bring a water kettle to a boiling point.

Compare this to the amount of energy produced by the sun, which is 380 billion TJ per second. That's more than enough energy to instantly boil all the oceans on planet earth.

There's no type of foreseeable human technology that will ever produce more energy than a Dyson sphere. That's why the Dyson sphere is considered to be some holy grail of technology / energy production.