Here's why:
* Transitive verbs require a direct object. The verb "think" in this sentence does not have a direct object. It's not clear *what* most people are thinking *of*.
* Intransitive verbs do not require a direct object. The verb "think" in this sentence is used intransitively, meaning it expresses an action or state of being without affecting a direct object.
To make the sentence transitive, you could add a direct object:
* "Most people think of bamboo as a tree or bush because it looks like one."
* "Most people think of bamboo as a tree or bush when they see it growing tall and straight."
These sentences now have direct objects ("because it looks like one" and "when they see it growing tall and straight") that receive the action of the verb "think."