Why can't we theoretically determine the rate order for reactions? I understand that in order to determine the overall order of the reaction (both as a whole and with respect to specific species) experimental evidence is needed. But I don't quite understand why? What part of this makes predicting and modelling these reactions so hard when there is moderate understanding about all of the components (e.g. the reactivity of different species, the steps in the reaction, the nature of the collisions etc.)?

Any help would be appreciated.