Final answer:
Unfortunately, the provided text excerpts do not specify why Mami won't buy Mrs. Robertson a Chia Pet. To answer this question, more context from the text is needed to ascertain Mami's reasoning.
Step-by-step explanation:
The question seems to be related to a reading comprehension exercise, asking for the motivation behind a character's decision not to purchase a gift, specifically a Chia Pet. However, the excerpts provided from the text, which mention characters named McTeague and Daisy who are discussing unexpected topics, do not give any direct insight into Mami's reasoning for not buying Mrs. Robertson a Chia Pet.
In instances like these, it's important to carefully read the text for context clues about character interactions and motivations that could explain Mami's decision. Without more information or a specific passage that highlights Mami's reasons, we can only speculate about why she would choose not to buy the gift. We would need more direct information from the text that clarifies Mami's reasons, such as her knowledge of what a Chia Pet is, its cost, Mrs. Robertson's desires, or her deservingness of a present.