If you want to read a long-ass book that tries to answer that question, check out Political Order and Political Decay by Francis Fukuyama.
In brief, he puts much of the blame on the US's incomplete separation of powers and a tendency towards nepotism (which has been prevalent through most of US history, with the exception of the early 1900s to the 1970s, according to him)
So the US has an often ineffectual executive branch, further saddled with an activist legislature that likes to meddle in its affairs. Yet, the legislature is hobbled by a patchwork of largely independent elected or appointed judges that vastly complicate the enforcement and interpretation of laws.
According to him, it is less of a question of how much the government does and more a question of how it does it. He tends to favor centralized parliamentary systems (reduce conflict between executive and legislative branches) with powerful, efficient, independent bureaucracies (leave day to day operations to a professional class and minimize political appointees). Yet such a system would be hugely unpopular to wide swaths of the American public, who traditionally prefer a degree of decentralization and elected officials as opposed to appointed bureaucrats.
Sure thing! I should point out that's the first book of a two-part series, although both books are well-regarded.
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推荐两本书。亨廷顿和学生福山也算一脉相承。老是拿民主非民主划分阵营,幼稚且可笑。去正经一点的大学,严肃的学一学当代学者对这个问题的看法,不至于被媒体和政客拿来当枪使。