Here's why:
* "Pleased" suggests a mild level of happiness.
* "Really" intensifies the happiness but doesn't change its fundamental level.
* "Extremely" pushes the happiness to a high level but doesn't necessarily signify the absolute peak of joy.
By combining these words, the phrase creates a strong sense of happiness, but it's not a literal statement. The speaker is using hyperbole to communicate that they are very happy, even if they aren't *literally* at the peak of their emotional capacity.
This type of language is common in everyday speech and can add humor, emphasis, or even irony to a conversation.