Expressions like "eat your vegetables" and "life is short" are also trite. But the purpose of folk wisdom isn't to shock and awe with fresh insights. The purpose is to remind people of what they already know to be true. That way we steer our behavior more towards the kind of life we want to live.
Your reply is simple, yet profound. Answers to the question of "why" must be understood.
Before retiring, I trained new hires, the age gaps were sometimes very interesting. The younger they were, the more they wanted to know. (obviously their experience level was pretty low.) I used adages to bring a point home when explaining a difficult concept. It worked every time.
Firstly, I'd argue it's only a tautology when you phrase it in a particular way (and take some knowledge about how the brain works as "given"). The point isn't that the effect happens at all, it's about the degree to which even small instances influence who we are.
But more importantly, we are creatures of habit before we are creatures of intellect. Stating things like "you are the sum of your experiences" isn't necessarily to reveal new and profound information, it is to emphasize a fact of existence that can easily become overlooked or underappreciated. Especially as we get on in the years and more and more of our behavior is on autopilot. There is value in reminding people to look for potential blindspots and providing some motivation a more careful consideration of the aspects of your life that have become unconscious even if the message comes in a trite or tautological package.
It's like people who have been stuck for the past 5 years spending 2+ hours every day scrolling a social media feeds littered with bad news, snapshots of arguments, and fake representations of the world but can't make a connection between this maladaptive behavior and the growing sense of cynicism and existential malaise that's impacting their happiness. Sometimes it is helpful to hear this kind of stuff (assuming it actually gets through) even though it should be obvious.
