A psychological reason for asserting such wild generalizations is the desire to attract attention.No one pays much attenetion to such undramatic statements as "some women are mysterious,"or "some politicians are no better than they ought to be."
What does the sentence in bold really mean? What do you think the message behind it is?
"some politicians are no better than they ought to be," is an undramatic statement and is very unlikely to elicit intense feelings from the reader.
If you say instead, "all politicans cheat and only look out for number one," that would be more forcible while basically conveying the same idea. Rhetoric counts.