There are practical considerations: you can't collect all the light, you can't focus the light down to an arbitrarily small size, and as the target heats up, it will radiate more strongly. Every ray that comes in must have an identical path leading away, and in the best-case scenario, where the target "sees" the sun everywhere, due to the massive mirror coverage, these will equilibrate when the temperatures are the same. In the less-than-ideal case, the target "sees" a colder reservoir somewhere, and radiates more energy wherever that happens, and keeps the target temperature below the source temperature.
The overall principle at play here is the second law of thermodynamics, which tells us that you cannot spontaneously transfer heat from a colder object to a warmer object.