Why is the word 'is' not considered as a preposition?

Question

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

Answer 1

In many ways "is" functions in a way similar to a preposition . The verb "to be" in its various forms has special functions unlike those of any other verb. None the less, it is a verb. In general English grammar does not use the label preposition for words that form another part of speech, and in particular, not for verbs.

Edit Note that nowhere in the (now) linked article is "is" or any of the other forms of "be" or any other verb listed as a preposition. Prepositions can be used to indicate a relationship, but not usually the relationship of identity or equality "This is that".

This distinction may not be fully logical, but on the other hand all prepositions normally so defined work as parts of sentences that have verbs other than the preposition itself. If we define "is" in "he is honest" to be a preposition, we would have a verbless sentence. Many verbs define or express a relationship, so the above definition of a preposition cannot be complete and exclusive.

Answer 2

Basic Grammar:

English word classes: taken from Cambridge Grammar online

Major word classes English has four major word classes: nouns, verbs, adjectives and adverbs. They have many thousands of members, and new nouns, verbs, adjectives and adverbs are often created. Nouns are the most common type of word, followed by verbs. Adjectives are less common and adverbs are even less common.

Many words belong to more than one word class. For example, book can be used as a noun or as a verb; fast can be used as an adjective or an adverb:

Other word classes The other word classes include prepositions, pronouns, determiners, conjunctions and interjections.

Prepositions Prepositions describe the relationship between words from the major word classes. They include words such as at, in, on, across, behind, for:

We went to the top of the mountain. (to describes the relationship between went and top; of describes the relationship between top and mountain)

Are you ready for lunch yet? (for describes the relationship between ready and lunch)

English word classes

He is honest. he=pronoun is=a verb honest=an adjective

There is no preposition in that sentence. There is a relationship between the subject and the predicate there but there is no preposition.