system
Given the following post, respond as if you're commenting on Reddit:
human
[Guardians of the Galaxy] Why is Groot limited to "I am Groot", no more, no less, without any permutations of that phoneme inventory?

Why can't he say words like "root" or "grew" or "tomb" or "ma'am"?
gpt
Maximus the Mad once claimed that "the mature dendritic form" of Groot's sound-producing organs is stiff and heavyweight.  Consequently, he can only produce a few baseline sounds.  However, there's enough nuance to how Groot produces these sounds that he can convey enormous amounts of meaning.  Not many are practiced in picking up on this meaning, though, and so Groot's (alleged) genius often falls on uncomprehending ears.

Maximus also claims (in answer to your question) that Groot's meaning is contained in the "sigh of breeze" beneath his repeated phrase.  This suggests that "I am Groot" is just the tip of the iceberg, and his expressed meaning is carried more alongside the phrase than in it.  It may be that we rarely, if ever, hear other phonemes because "I am Groot" is just a frame over which is stretched the true vast canvass of his language.