Because you do not have direct access to the FADEC.
Access is via the FMC which "talks" to the FADEC.
There are many levels of redundancy for all these calculations,
ACARS RTOW calculations are rejected if rubbish is input.
Similarly, the FMC will reject many inappropriate inputs.
The old fashioned paper calculations mentioned above seemed to work without FADEC and other modern electronic gizmos.
When the various electronic calculators are unavailable, there is usually a paper backup method available with tables for weights/temperatures/pressures and wind components for the available runways.
[This was in reply to another question posted by SLF...which seems to have disappeared !
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