Why are offenders never charged with obstruction.?

Usually, the charges they eventually face carry higher penalties than the obstruction charge. The police will always go for the charge that carries the higher penalty especially if they can eventually get a confession. If the offender persists in lying and brings in someone else to back up their story, they can be charged with perverting the course of justice. That charge almost always results in a jail term, whereas the lesser charge of obstruction does not.