One thing to note regarding the various intensities of turbulence (light, moderate, severe, extreme) --
What may feel like "severe" turbulence to someone sitting in the cabin, especially in the aft sections, as you've mentioned, may in fact only be "moderate" turbulence. There are defined parameters for each level of turbulence/chop.
Obviously, I don't know what you've experienced personally, but most people don't experience "severe" turbulence on a routine basis, as operators take all possible steps to avoid it.
Moderate turbulence encounters are much more common, (although they may feel quite "severe" from an individual/subjective viewpoint). Even with moderate turbulence, most operators will attempt to find smoother altitudes, if other altitudes are reported to be better, and if they're available. However, sometimes there is no smoother altitude, so you simply fly (at the lower turbulence penetration speed, as explained in by tunneler and john_t) until you've passed through the area.