Why many excel migration Projects fail ?

In last 3 years, i witnessed 2 large projects to migrate excel to erp system failed in separate corporations. First one - aim was to move the process to oracle erp. The excel file was huge, 100s of unique large formulas and dozen and dozen layer of depencies -still managed to code in new system. After deployment - business was not confident of the output as they could not figure out the full cover of test cases. So the project delivered - but not used. Second was the move to sap. Expensive programmers and analysts pulled from big consultancy form. After 4 weeks it was deemed too complex to map the full picture of excel and resource demand almost doubled. Business decided its not in priority for expense and got canned. Just sharing experience that how important it is to document the major flow and changes in excel to avoid being in unescaping pit.