Don't do that. Yes there is a reason. The mulch has not decomposed. As you have experienced, the decomposition process for wood takes a long time. It also requires a large nitrogen source, like air. If you mix the wood mulch in with soil, that deprives the decomposing wood from the easy N source and forces it to "rob" nitrogen from every other source. You would be sentencing yourself to 10 years of poor looking grass because no amount of fertilizer would be enough to satisfy both the decomposing wood and the lawn. It would be much better to use plain sand or pulverized topsoil to fill low spots.