Here's why:
* Abstract nouns refer to concepts, qualities, ideas, or feelings that cannot be touched or physically experienced.
* Concrete nouns refer to things that can be touched or physically experienced.
Similarity is a concept. It is the state of being alike or having a resemblance. You can't touch or hold "similarity" in your hand.
Here are some other examples of abstract nouns:
* Happiness
* Courage
* Justice
* Love
* Freedom