France has very strict privacy and confidentiality laws that apply to the authorities as much as, if not more than to private companies and individuals. One department can’t simply ask another department to share personal information.
I remember that when healthcare switched from CMU to PUMA and URSSAF was charged with calculating cotisations based on information about income submitted on the income declaration submitted to the tax office (previously inactifs had had to fill in a separate income declaration to URSSAF for this purpose), the Senat had to debate and approve the transfer of data from the fisc to URSSAF.
Additionally, the tax office cannot simply demand taxes if a person did not meet the criteria for tax residency. Holding a WA CdS is not, in itself, a criterion for tax residency. It may be that a person who has a WACdS and has not filed taxes has committed fraud, but they have not committed tax fraud so it is not the fisc’s business. The penalty for making a fraudulent application for a CdS is a different matter altogether.