Here's why:
* Past participles can function as adjectives, describing a state or condition. For example: "The engrossed reader didn't notice the time passing."
* Past participles can also be used in perfect tenses (present perfect, past perfect, future perfect). For example: "I have engrossed myself in this book."
So, "engrossed" can be both an adjective and part of a verb phrase, depending on its context.