Here's why:
* Subject: A complete sentence needs a subject, which is who or what is performing the action. "Bloom" doesn't tell us who or what is blooming.
* Verb: While "bloom" is a verb, it's not conjugated in a way that indicates a complete action.
To make it a complete sentence, you need to add a subject and potentially an object, depending on the intended meaning.
For example:
* The flowers bloom. (subject: flowers, verb: bloom)
* The rose blooms beautifully. (subject: rose, verb: blooms, object: beautifully)