Why aren't mitochondria and plastids considered symbiotes of eukaryotic cells?
I would say it has to do with the amount of mitochondrial or sequence that has been transferred to the host genome. As a consequence of all this information stored in the host genome, mitochondria cannot reproduce without the host. In this way, they are not their own organisms, but rather organelles.  

Over evolutionary time, the line between organelle and intracellular endosymbiont gets blurry, but in their current state, mitochondria are organelles.

The same goes for for plastids, in general.