Here's why:
* Abstract nouns represent ideas, qualities, or concepts that cannot be touched or seen. Examples: honesty, happiness, freedom.
* Concrete nouns represent things that can be touched or seen. Examples: table, tree, book.
"Doctor" refers to a person, a specific role or profession. It's a concrete noun.
However, there are some cases where "doctor" might be used in a way that could be considered abstract. For example, if you say "The doctor is coming to the rescue," you're using "doctor" to represent the concept of medical help or expertise, which could be argued to be more abstract.