RDS is a managed service. This is a code repo. It is useless until you deploy it at which point... it's no longer free. Then you have to manage it yourself and deal with the overhead yourself.
This is like saying I can fire my gardener because you have a free gardener alternative, and then handing me a pair of scissors. I now have all the problems that I paid the Gardener to solve.
Likewise, we pay AWS to manage all the headaches that deploying this would introduce. And trust me, if you are running databases at scale in production, then RDS feels very affordable considering the problems it solves.