A “career” in the Reserves may be different than what you think. Many who appear to be on active duty are reservists on active duty, not regular army. RA are guaranteed jobs, reservists on active duty not so much. Then, the ones that meet monthly and for 2 weeks in the summer are subject always to being ordered to active duty. So yeah, it is actually a career.
The days of extended active duty for Reserve Component soldiers is coming to an end. A small percentage will qualify for full time positions in the Guard and Reserve in the Army, the Marines fill those positions with Regular Marines. Entry into and promotion within the full time Reserve force is highly political, exclusion from certain combat units has little to nothing to do with it.