Ultimately, they believe what they believe because they are economic illiterates. A lot of it is emotional and feelings-based rather than fact and reality-based. Billionaires should be taxed more because they don't "need" or "deserve" that much money. Who gets to determine that? Well they do, of course. Before you know it, nearly everyone is paying this additional tax that at one point only billionaires were paying. Just like the AMT and the income tax itself.
Many of them have a false or childlike understanding of money or just simply how the world works. I've seen multiple places on social media where a leftist has said something like "Bill Gates retired from Microsoft in 2006, yet since then he's gotten even richer. That shouldn't be able to happen. How is that possible? Any additional money he makes while not working should just be taxed." It's as if they've never heard of the concept of ownership in a company, compound interest, or anything having to do with capital and investing.
A lot of what it boils down to is that many on the left see economics as a zero-sum game. There's a belief that for somebody to be rich, then somebody else (maybe even a ton of somebody elses) must be poor. In order for Bill Gates to have a $100b net worth, then 10 million people must be poor. Therefore we should change how the pie is sliced so it is more "fair," not try and make the pie bigger.