Why do so many people think Democrats are a bunch of evil Communists?

It's the result of propaganda.  

Not only do many people think Democrats are a bunch of evil communists, others think Republicans are a bunch of evil fascists. The Republican Party and the Democratic Party find, to stay in power, they need to keep funds and donations coming in. To do that, they need to convince a lot of their supporters that policies of the other party are terrible, and will be destructive of American society. As one party amps up the fear level, the response of the other party is to amp up the fear level on their side.

Comment: 

What is worse, enemies of the United States can use this against us. They can play both sides, feeding fears of the left to those on the right, and feeding fears of the right to those on the left. They can promote and feed conflict, and then say to those in their own country "SEE! Democracy doesn't work." This also reduces the influence the United States has abroad, giving them more influence.

I think  that's just an exaggeration in an environment  of  freedom of speech. 

Because they believe the words of a proven pathological liar and his cronies like McConnell, McCarthy and Graham!

Communist, socialist, the Great Reset, who cares? They are Marxist Kleptocrats who stole the elections and they are starting to lose their camouflage. The wilderness they chose to hide in is being lit up to expose them. They are losing their ability to corrupt the courts, and people who push the effort are tireless. 

Former Maricopa County Recorder Fontes in Panic Mode - He calls Arizona Republicans Seditionists.

https://www.youtube.com/watch?v=sWcXrSHWG6I

The voices in your head don't count as "so many people."

People think communism means 'when the government does something' and it's far easier to just hurl insults than actually discuss the issues.

30 years of right wing media brainwashing stupid people. Keep in mind none of those people know what communism actually is.

"So many people" do not think Democrats are a bunch of evil Communists. 

Only a small group of very stupid people who have no idea what simple words mean think that. 

The simple words are "Democrat", "Communist", "Liberal", "Socialist" to name a few.  

The people in your trailer park do not represent the majority in the country.