Why are detritivores, decomposers and omnivores not assigned trophic levels

The reason is because detritivores and decomposers and omnivores are multilevel food chain consumers in an ecosystem making them not qualified to be assigned to a given trophic level

Explanation:

An organism is placed in a trophic level based the level it is from where the food chain starts with the start of the food chain having a level of 1 for the food producers such including plants and algae

Level 2 organisms are called primary consumers and consists of organism that eat only plants known as herbivores such as cattle

Level 3 organisms are called secondary consumers and consists of animal eating organism known as carnivores such as foxes  etc.  

Level 4 organisms are called tertiary consumers  

Level 5 organisms are the a p e x p r e d a t o r s

Detritivores and decomposers such as bacteria, earthworm flies fungi consume the wastes produced at all trophic levels, therefore they cannot be assigned to a particular trophic level

Similarly, omnivores consumes both plants at level 2 and animals at level 3 making it ambiguous to assign them to one particular trophic level.

coal is not sustainable because it will one day run out.