Basically can someone explain why the molecularity cannot be determined from the rate law?
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The coefficients only correspond to the rate law coefficients if the reaction is elementary. Elementary reactions are ones where the reagent(s) combine through a mechanism implied by the reaction. These are simple. Many reactions aren't as simple and have multiple steps.
Check out this link, example two. The reaction is actually comprised of two reactions where the slow step determines the rate coefficients.
As you can see, you cannot just read the equation and get the coefficients. One can only determine whether a reaction is elementary (i.e. find its rate law coefficients) by experimentation.
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$\begingroup$ I might add that even if you have the rate law coefficients it is not always possible to infer the elementary reaction steps from it, there might be multiple possible paths that result in the same rate law. For further distinguishing between the paths you have to use in-situ techniques that can monitor intermediates while the reaction is going on $\endgroup$– MichielCommented Mar 27, 2014 at 6:22