I can give this answer from an SMB perspective. I feel like I am probably really close to the majority of SMB that try and deploy AD, specifically "because we already have it". In my implementation, we had two locations and two hosts initially, so it seemed a no brainer to use to DC's. However, I would guess that a lot of people that are setting up AD don't really know what AD even actually does. I have confused other things built into windows or NTFS with being AD simply because I manage the from my DC. I am talking about things like group policy, security groups, file permissions, etc. I would bet that the majority of SMB that deploy AD do so because they want to leverage these things and because owning windows servers gives them access to AD which is included.
With all that said, I know that AD's simplicity can be deceiving and there is a high chance that just because you have two DC, it doesn't mean that you have them configured correctly, I know that I don't.
How often does the real SMB actually have people that already know AD and what it actually does, and know that there are any other options in a windows ecosystem? I didn't even know I had a choice until recently.