In my experience the reason this is done is because customer acquisition is manual and the "schedule a demo" button is used for lead generation, and the reason it's all manual is because onboarding/support is high touch and you're not sure who the customers are yet.

Free demos are my nightmare. I really don't have that much time to split between fixing existing issues, adding critical features for paying customers, and supporting or troubleshooting for users that aren't paying anything. That's not to say at a point in the future the organization could have the capacity for a sick free trial, it's just that free trials cost money.