Because in the typical American mind a thee-not-me-mentality is very prevalent. They are the sole protagonists of their own lives and they believe they have invincible plot armor like the protagonists in the action movies.
That means people believe the police having Gestapo-capabilities will only ever hurt others (who must then have deserved it!) and never them.
That a free democracy is a finely balanced system between the power of the people and the powers of the three branches isn't something they ever consider. That capabilities can breed their own perverse incentives and a culture of disservice is a few complexity levels further up the tree still.
That means people believe the police having Gestapo-capabilities will only ever hurt others (who must then have deserved it!) and never them.
That a free democracy is a finely balanced system between the power of the people and the powers of the three branches isn't something they ever consider. That capabilities can breed their own perverse incentives and a culture of disservice is a few complexity levels further up the tree still.