Why don't self pollinated crops show inbreeding depression?
Yes, self pollination does lead to inbreeding depression:


  However, we found substantial inbreeding depression, with seeds per fruit, progeny survival and progeny growth being lower after self- than after cross-pollination. Progeny arising from self-pollination also had a higher frequency of certain traits ... which were associated with lower rates of survival. ... high inbreeding depression has also been reported in other self-compatible trees ...


--Self-pollination and inbreeding depression in Acacia dealbata: Can selfing promote invasion in trees?


  Inbreeding depression is one of the leading factors preventing the evolution of self-fertilization in plants. 


--Inbreeding depression in self-incompatible and self-compatible populations of Leavenworthia alabamica.


  The strategy for breeding F1 hybrid squash is to develop parental lines through self-pollination. However, it increases plant mean homozygosis, which is not the natural genetic state of a cross-pollinated species, and can cause "inbreeding depression". 


--Depression by inbreeding after four sucessive self-pollination squash generations

So why don't all crops that self-pollinate show decreases in productivity? In part, they do, but the reduction in productivity caused by inbreeding is outweighed by the increases in productivity caused by inbreeding (i.e. selection for productive traits via inbreeding leads to both positive and negative results, and the total outcome is the sum of the positive and negative effects). 

And partly because the deleterious effects of inbreeding can be partially overcome through "purging". The negative genes that lead to inbreeding depression when homozygous can be removed from a population as the affected individuals die (or fail to breed) and take their negative genes with them. Purging is most common when a population has a moderate degree of inbreeding over a longish time, which is what you'd expect with a crop that typically self-pollinates. It's an effect in many but not all common crops:


  Several plant species, E. g alfalfa (M. sativa) carrot (D. carota) , hayfield, tarweed etc show very high inbreeding depression. ... Many crops species, such as maize, jowar, bajara etc. shows moderate inbreeding depression. ... Several crop plants, E. g onion (A. cepa), many cucurbits, rye (S. cereale), sunflower (Hannus), hemp etc show only a small degree of inbreeding depression. Only a small proportion of the plants show lethal or subvital characteristics. The loss in vigour and fertility is small; rarely a line cannot be maintained due to poor fertility. The reduction in yield due to inbreeding is small or absent. Some of the inbreds lines may yields as much as the open pollinated varieties from which they were developed.


--Degree of Inbreeding Depression