On the surface, you are right. However you often still need custom code to handle the business logic. If a new customer joins you have spin up a new instance. If a customer leaves then you have to delete the instance. How do you automate backup and restore? Is there a central repo for record keeping and how does that updated?

Sometimes multi-tenancy is a simpler solution because no instance fleet management code needs to be written. With everything there are downsides to multi-tenancy too so you have to weigh the pros and cons for the specific project.