Apparently a lot of folks
do reuse soil. That link is a RIU thread from '10, I'm sure there are more, but you'll have to do your own searches if it interests you. According to my five minutes of research, (I'm an expert now, right?) the best way to reuse is to amend it with whatever nutrients it now lacks, mix it up a bit and let it 'age'. The link has good details.
Myself, I'm not going to bother. I don't grow that much. I'm an indoor gardener and spend enough time on this hobby as it is. I'm not going to try to tweak dirt to save a couple of bucks. I don't throw the old stuff away, I just use it in the yard and garden mixed with the dirt that's out there.