First, this made me giggle because I run into that attitude all the time. "You're hosting things on a SERVER? Why would anyone do THAT? Heck, you should be putting everything in serverless and avoiding even the vague possibility that you would have to touch anything so degrading and low-class as an operating system. Systems administration? Who does that?"
In all seriousness, however, the decision (likely) has very little to do with that. They're most likely not hosting in the cloud because the current CA/Browser Forum rules around the operation of public CAs effectively don't permit cloud hosting. That's a work in progress, but for the time being, the actual CA infrastructure can't be hosted in the cloud due to security and auditability requirements.
In all seriousness, however, the decision (likely) has very little to do with that. They're most likely not hosting in the cloud because the current CA/Browser Forum rules around the operation of public CAs effectively don't permit cloud hosting. That's a work in progress, but for the time being, the actual CA infrastructure can't be hosted in the cloud due to security and auditability requirements.