Why is alcohol used to prepare extracts, such as mint extract?
Simple. The compounds that give mint its minty flavor are alcohol soluble, so with alcohol present you can "extract" them.
There are other things use can use, propylene glycol is one. Basically you need a solvent that can also serve as a carrier for the flavor you want. Water doesn't work, most flavors are not water soluble. Many are oil soluble, but using oil gives you a different product entirely, as does using vinegar for the acid, which is also a solvent.
If your final product is high in alcohol, propylene glycol or acid, it will also be well protected against quick spoilage.