Here's why:

* Substrate molecules exist independently: Substrate molecules are the reactants in a chemical reaction. They are present in the environment and exist regardless of whether an enzyme is present.

* Enzymes catalyze reactions: Enzymes act as catalysts, meaning they speed up reactions but do not change the overall outcome of the reaction. They provide an alternative reaction pathway with lower activation energy.

* Reactions can occur without enzymes: While enzymes make reactions happen much faster, the reactions can still occur without enzymes, just at a much slower rate. This rate might be too slow to be significant biologically.

Think of it this way:

* A substrate molecule is like a log.

* An enzyme is like an axe.

* The reaction is chopping the log into smaller pieces.

The log (substrate) exists whether or not there's an axe (enzyme) around. The axe (enzyme) makes chopping the log (reaction) much faster and easier, but the log can still be chopped (reaction can still occur) without an axe, just much more slowly.