Why aren't organelles considered alive?
The definition of life is a controversy in itself, and as it is simply a word that can be understood by everyone however they wish, there is no "correct" definition. It is thus not really possible to give a "correct" answer to your question, but here are a list of things you may want to consider:


Replication of DNA is probably the most agreed common feature to all life on earth. Most organelles do not contain their own DNA but are simply lipid and protein constructs.
Independence is a difficult criterion of life, as parasites require a host for reproduction. However, while the organism may require a host, its cells individually can replicate without a host.
Cells are able to replicate when removed from the body - under the right conditions. (We are getting better and better at culturing all sorts of cell types, for example to avoid having to use animals for research.) Most organelles are not able to replicate under any conditions.
In nature, there are single living cells. In fact, the majority of organisms on earth consist of one cell only. There are no organelles living in nature.


Mitochondria and plastids make a complicated exception to these, as they are originally derived from bacteria themselves. It is indeed debatable whether they could be considered alive, as they possess DNA and it is replicated in order for them to divide, like individual cells do. However, the machinery required is not produced by themselves but rather by their "host" cell, making them subject of a similar argument as viruses.