Here's why:
* Prepositions show the relationship between a noun or pronoun and another word in the sentence. In a comparison, "like" shows the relationship between the noun being compared and the thing it is being compared to.
* Example: "The cat is like a fluffy cloud." Here, "like" shows the relationship between "cat" and "fluffy cloud".
Note: While "like" can also be used as an adjective or a verb, in a comparison, it functions primarily as a preposition.