Because an airline or TRTO may choose to be 'zero flight time' or not. If they are authorised ZFT then everything may be done in the sim. If not, the number of circuits required in a real aircraft is determined by the experience of the individual. If, for example, he has 737 time and is converting to the fabulous Airbus, then only four are required. If he comes from a KingAir, then six (I think).