The truth in health and fitness is that it's more like an art than a science.
Amen. The art of adaptation, and knowing how to help someone adapt is hard. It takes years and years to learn your own body, and most people never do it. Science can be a tool, but science does not create artists who can use these tools to create amazing results.
Furthermore, training is much more than just structuring workouts. Many times we need to learn to speak their language, and show them how their goals require out methods. People have fears, doubts, insecurities, hesitations, misconceptions, etc. Helping them build mental strength and focus is just as important as giving them a list of exercises or foods. You also have to build trust.
I've been doing this for 25 years and I still don't feel 100% confident giving advice. I take it seriously, and always scrutinize every word I say 10x over.