In the elementary steps of a reaction, why do we assume that the coefficients of a reactant correspond to its order?
If a reaction has just one step, it's named an elementary reaction. In this reaction all of reactant directly participate on reaction and effectively collide with each other. So, the order of reaction corresponds to its coefficient, but if a reaction is not a elementary reaction, there isn't any reason for colliding all reactants with each other. For example, consider the following overall reaction:

$$\ce{A + B + D -&gt; F}$$

Assume the corresponding mechanism is constituted of two elementary reactions:

$$\ce{A + B -&gt; C}$$
$$\ce{C + D -&gt; F}$$

As shown above, there is no need to collide A with D or A with C ... Then, in these type of reactions coefficients are not corresponded to order of reaction .