The answer was that they simply didn't trust GCP or AWS or Azure to see their data and know how much silly money they were making in the niche industry they almost completely monopolize.
I recently interviewed with a lower-case-m megacorp in a similar situation and they host on-prem for the same reason, at great expense and hassle in facilities all over the country.
Seems like theres room in the market for some kind of an On Prem Private Cloud Stack that emulates GCP/AWS etc but locally maybe?
Cloud-In-A-Box anyone?