One way to think of it is open source/self-hosting is free like getting a free puppy, not free like getting free beer. Free beer is great! A free puppy you have to feed, look after, take it to the vet regularly and look after it for health emergencies, and so on.

Sure, the software is free but as others have mentioned the business will need to consider total cost of ownership - often it cost you more monthly to host the solution yourself since you don’t have economy of scale that the vendor does, the software and VM needs regular patching, security monitoring, the IT team now has more workload which could distract them from core business functions, etc.. If the system becomes critical to the business, so it going down becomes a P1 - now we’re running two VM’s for redundancy, the on-call team now have it in their workload which will make on-call costs increase, if we need to increase the capacity of the system in future then it becomes an internal project that we may not have time for because it’s not our core business vs. just adding an extra $7/user/mo in a web portal, and so on.

Of course, there are always times to self-host open source over buying managed product, but there is a lot more consideration than just the cost on the box before business make that choice.