Many metabolic reactions and processes cannot occur simultaneously since they require specific conditions, such as different pH levels and specific localized environments to occur with particular molecules and enzymes. 

Cell compartmentalization enhances cell efficiency by permitting the occurrence of metabolic processes at the same time, (which is otherwise not possible) and minimizing the waste of molecules. Compartmentalization also prevents potentially dangerous molecules from roaming freely within the cell.

Substrate competition is another reason metabolic processes can’t occur at the same time. Two distinct substrates may compete for the active site of the same enzyme. Two distinct enzymes may compete for the same substrate as well.