Why must there be an object after a preposition in English?

Because the object of a preposition is usually expressed by parts of speech or structures such as nouns, pronouns, gerunds, and noun clauses. However, parts of speech or structures such as adjectives, adverbs, prepositional phrases, and verb infinitives can also serve as objects of prepositions under certain circumstances.

Types:

Simple prepositions include in, on, with, by, for, at, about, under, of, to, etc.

Synthetic prepositions include into, within, throughout, inside, outside, without and other prepositions composed of two words.

Overlapping prepositions include from among, from behind, until after, at about, at about.