I am not sure about the “build it yourself is expensive” argument. If you were non-technical, you probably shouldn’t try to build it yourself, but for technical folks, that’s not a stretch.
Fair point — a technical person can absolutely wire up pre-signed URLs and a basic UI. The target here is more the IT lead who gets the ticket from finance or legal: “we need access to these files on S3.” They could build it, but it’s a week of work, then ongoing maintenance, auth, permissions, audit logging, SSO… BucketDrive is a CloudFormation stack that gives them all of that in 10 minutes, running serverless in their own account. The “build vs buy” math changes when the person asking isn’t the one building.