Why is it that leftists in the US who are trying to destroy capitalism think that they will be so much better off under communism?

First, there is no evidence that "leftists" are the same as "communists." This is name-calling that often occurs in politics; sadly, some right-wing media sources and certain conservative politicians seem to believe that any criticism of big business makes one a communist. That is not true. Further, there is also no evidence that "leftists" want to destroy capitalism. On the other hand, many left-leaning Americans (some of whom are Democrats) and even a few moderate Republicans believe that capitalism is broken, that a small group of people are getting very, very rich thanks in large part to loopholes in current tax law. These people do not hate capitalism at all-- they simply want to see its excesses corrected.

To be clear, many people whose politics lean left are upset because they see too much income inequality in America today, and they believe the top 1% have benefited greatly, even during our difficult economic times, while everyone else is struggling. They believe the tax code is unfair and that currently, giant corporations get tax breaks they do not deserve-- for example, there is evidence that many corporations have laid off American workers and moved those jobs overseas to third-world countries; and some corporations have hidden assets in shell accounts in foreign countries so that they do not pay their fair share of taxes. Whether or not you think that corporations who do such things are within their rights, those people who criticize what they regard as corporate greed are certainly not communists.