But someone in their org has to set up the spreadsheet, and more importantly, take the blame if the numbers don’t match Management expectations, even if they are correct.
By outsourcing this to the software vendor, they also can outsource responsibility. Instead of Management leaning on the team to conjure “better” numbers, the team can say “We get these numbers from this independant vendor outside the company, who has all the info and is totally impartial, nudge nudge, wink wink”. Questioning the numbers then becomes questioning the financial decision to employ the vendor in the first place. Just so much easier to do what the nice computer tells you to do.