We still use paper RTOW charts with corrections for environmental conditions. For example, a low QNH requires a reduction in weight and having ice protection on requires a reduction in weight. By allowing the software to provide RTOWs that are higher than the MTOW you can apply corrections and still go at MTOW. If the RTOW was capped at MTOW then any performance decreasing adjustments would result in an RTOW that was below MTOW even though it didn't need to be.