Difference is, most models can be validated against reality. I might model a car crash impact in LS-DYNA, then I eventually crash the real thing and compare. Unless you have a spare Earth lying around and a few centuries, then climate modellers can't do that. Instead they have maybe 80 years of reliable temperature data at best (which is just one variable), on a planet 4 billion years old.

The radiative physics can tell us a lot about basic climate sensitivity, but modelling the feedbacks inherent in such a complex system must be near impossible. Saying that climate change is "simple radiative physics" is very misleading, it is a little bit like saying that if I kick a ball it will accelerate away from me and that is "basic Newtonian physics". Well yes, but I may be standing at the bottom of a steep valley, in which case the larger system exerts a negative feedback, and the ball rolls back to my feet.