Besides, at enterprise scale, how do you trust internal teams? It could all be security theater and they aren't delivering on their promises as well.
Some parts of it maybe do. Some others, like multiple different Azure teams, don't even think about anything resembling security, or there wouldn't have been multiple critical and trivially exploitable security vulnerabilities on Azure in the last year only. (If you don't know them, please read up on them. Security is hard, but in those cases nobody even pretended to try!)
The managers who want remote attestation aren't the people implementing it. They either pay someone else to do it, or they pay someone else to do it. The difference between paying a third-party company and an employee is that employees are more expensive, because the costs aren't amortized over other customers who want the same stuff. Why would they be more trustworthy? Why would they be better at it? Why would it be any less likely to be hacked if you did it at your company than if you outsourced it?