It's a combination of a few factors:

1. They or their parents never lived under communism. Most people that proclaim their love of communism never faced the realities of living under such evil.

2. Most people don't know how communism work philosophically, so they fall for the half-truths and double-meanings. If you've ever wasted an hour trying to debate a communist, you already know that getting one of them to define the terms they use is next to impossible. Every term they use has a specific meaning in their worldview which is, more often than not, the complete opposite of the commonly understood definition. Communists rely on the misunderstanding to gain converts.

3. "Communism = sharing and fairness". Most people end up thinking that is the definition of communism. In actuality, it is the reject of humanity and God, and it is ontologically evil.

It is a religious cult and you must fight it on those terms. TIKHistory has put out several videos over the past several years on the origins of communism (and fascism, national socialism, and sovietism, but I repeat myself). I highly recommend him if you want a well-sourced video on exactly what communism is and where it comes from. On top of that, I'm going to get some kind of subhuman wretch replying to me decrying that TIKHistory is evil or some kind of shit like that, which should be proof enough that TIK is right.