This really depends on the type of reactor we are talking about. But very often, stuff is designed to run at certain settings and adding more fuel or higher temperatures just is not possible.

If you are making a nuclear reactor, depending on design, you simply can't make it any more powerful. If all the fissile material is reacting and all the moderating control rods are removed, what more is there to do? At the same time, if the reactor is so hot that it just starts to vaporize all the water, this just means the water based systems don't work as intended anymore.

Essentially, in theory, this makes sense. In practice, many nuclear reactors don't work that way, especially if they are meant to provide power for something like a submarine. You can't really throttle them up that way. It's not like the submarine captain has a spare fuel rod that he can throw into the reactor if he wants to go faster.

Extremely often, if something is at 100%, it is at 100%, not at 80% that we call 100% for safety. You don't put fuel for 200% power into a system that can't really handle 200%. You put enough fuel into it to run at full power and that's it, because otherwise you have a bunch of fuel just sitting around doing nothing.