Why all drugs are not given through parenteral route?

Most take medications at home and do not have the training nor the equipment to do so.

Only drugs designed and intended for administration via a parenteral route should be taken that way. If drugs designed to be taken orally are administered parenterally the "user" could be harmed, and the medication may work very differently than intended.

In addition, most drugs are tested as orally administered medications. The way they are absorbed, how they are metabolized, and the dose and time to effect are all designed for oral administration.