I am reading through the blue grammar book and the following is the definition of prepositions-
A preposition is a word or set of words that indicates location (in, near, beside, on top of) or some other relationship between a noun or pronoun and other parts of the sentence (about, after, besides, instead of, in accordance with). A preposition isn't a preposition unless it goes with a related noun or pronoun, called the object of the preposition.
If we consider the sentence- "he is honest", 'is' shows the relationship between 'he' and 'honest', it tells us he has the quality of honesty, I understand why it is considered a verb but do not understand how it is not a preposition as it satisfies the definition above?
Link for the relevant passage- https://www.grammarbook.com/grammar/probPrep.asp